JP2021153293A - Electromechanical conversion element, ultrasonic transducer, ultrasonic probe, ultrasonic diagnostic device, and method for manufacturing electromechanical conversion element - Google Patents

Abstract

To provide an electromechanical conversion element that has a high frequency and high resolution and can ensure appropriate sound pressure and reception sensitivity.SOLUTION: In an ultrasonic transducer having a plurality of piezoelectric elements 20, the piezoelectric elements each have a substrate 11, a first electrode 21, a piezoelectric material 22, a second electrode 23, and a gap part 30 that is formed on the substrate 11 on the opposite side of the piezoelectric material 22 across the first electrode 21, which are sequentially laminated. The width Cw of the gap part 30 and the width Pw of the piezoelectric material 22 seen from the direction of sequential lamination satisfy the conditional expression (1): 0.65≤Pw/Cw≤0.95.SELECTED DRAWING: Figure 7

Description

The present invention relates to an electromechanical conversion element, an ultrasonic transducer, an ultrasonic probe, an ultrasonic diagnostic apparatus, and a method for manufacturing an electromechanical conversion element.

Transducers that vibrate a thin film to transmit and receive ultrasonic waves are being applied as inspection / measurement devices for medical diagnosis, industrial use, in-vehicle use, and marine use.
In particular, medical ultrasonic diagnostic equipment is widely used because it allows easy real-time observation of internal tissues.
Conventionally, as an electromechanical conversion element used for such an ultrasonic transducer, a ceramic lead zirconate titanate (PZT) called bulk has been known to be die-divided into individual pieces, but in recent years. Documents using semiconductor technologies such as PMUTs (Piezoelectric Micro-Machined Ultrasonic Transducers) and CMUTs (Capacitive Micro-Machined Ultrasonic Transducers) using piezo elements are known.

In particular, when PMUT is used, it is compact and thin because it can be manufactured and structured easily and can be operated at a relatively low voltage, in addition to achieving high resolution and high frequency by microfabrication. It is expected as a technology suitable for 2D array formation.

When trying to reduce the size, thickness, and frequency, the size of the electromechanical conversion element, which is each vibrating part of the ultrasonic transducer, is naturally as small and high as possible. It is preferable to make it. Further, it is preferable that the piezoelectric element constituting the electromechanical conversion element is also made as small as possible and has a high density.
However, it is known that the miniaturization of the piezoelectric element also causes a decrease in sound pressure during vibration and a decrease in reception sensitivity.
Therefore, it has been desired to develop an electromechanical conversion element capable of ensuring high frequency, high resolution, appropriate sound pressure and reception sensitivity.

In order to solve such a problem, in the electromechanical conversion element according to the present invention, the substrate, the first electrode, the piezoelectric body, the second electrode, and the first electrode, which are sequentially laminated, are sandwiched between them. The ratio of the width of the gap portion: Cw and the width of the piezoelectric body: Pw as viewed from the direction in which the piezoelectric body is sequentially laminated is a conditional expression. (1): Satisfying 0.65 ≦ Pw / Cw ≦ 0.95.

According to the present invention, it is possible to secure high frequency, high resolution, appropriate sound pressure and reception sensitivity.

It is a figure which shows an example of the structure of one form of the ultrasonic diagnostic apparatus which concerns on this invention. It is a figure which shows the structure of the control part of the ultrasonic diagnostic apparatus which concerns on this invention. It is a figure which shows the other structure of the ultrasonic diagnostic apparatus which concerns on this invention. It is a figure which shows an example of the structure of the ultrasonic probe shown in FIG. It is a figure which shows an example of the ultrasonic transducer shown in FIG. It is a figure which shows an example of the ultrasonic transducer shown in FIG. It is sectional drawing which shows an example of the piezoelectric element shown in FIG. It is a figure which shows an example of the manufacturing process of the ultrasonic transducer shown in FIG. It is sectional drawing at the time of vibration of the ultrasonic transducer shown in FIG. It is an enlarged view of the inflection point of the cross-sectional view shown in FIG. It is a figure which shows the frequency dependence of the void part which concerns on this invention. It is a figure which shows an example of the shape and film thickness of the piezoelectric body which concerns on this invention, and the output sound pressure. It is a figure which shows an example of the size of the piezoelectric body which concerns on this invention, and the output sound pressure. It is a figure which shows an example of the relationship between the size of the piezoelectric body which concerns on this invention, and the receiving sensitivity. It is a figure which shows an example of the size of the upper electrode and the output sound pressure which concerns on this invention. It is a figure which shows an example of the relationship between the size of the upper electrode and the receiving sensitivity which concerns on this invention. It is a figure which shows an example of the relationship between the size of the upper electrode and the capacity of a piezoelectric element which concerns on this invention. It is a figure which shows an example of the relationship between the film thickness of the piezoelectric material which concerns on this invention, and the breakdown withstand voltage. It is a figure which shows an example of the relationship between the film thickness of the piezoelectric body which concerns on this invention, and the capacity of a piezoelectric element. It is a figure which shows another example of the structure of the piezoelectric body which concerns on this invention. It is a figure which shows another example of the structure of the piezoelectric body shown in FIG. It is a figure which shows another example of the structure of the piezoelectric body which concerns on this invention. It is a figure which shows an example of the cross-sectional structure in the piezoelectric body shown in FIG. It is a figure which shows another example of the manufacturing method of the electromechanical conversion element which concerns on this invention.

Hereinafter, embodiments according to the present invention will be sequentially described with reference to the drawings. In the embodiment, the same functions and the same configurations are designated by the same reference numerals, and duplicate description will be omitted as appropriate. Drawings may be partially omitted or simplified to aid understanding of some configurations.

The ultrasonic diagnostic apparatus 10 according to the present invention has an ultrasonic probe 1 that oscillates ultrasonic waves toward the measurement object 9 and detects the vibration of the ultrasonic waves reflected on the measurement object 9, and an ultrasonic probe. It has a display unit 61 for visualizing and displaying a signal from the tentacle 1, an operation unit 62, and a control unit 63 as a terminal for controlling the ultrasonic probe 1.

As shown in FIG. 2, the control unit 63 electrifies the ultrasonic pulse generating unit 64 that generates a pulsed electric signal for generating the ultrasonic signal and the echo signal received from the ultrasonic probe 1. It has a conversion unit 65 that converts a signal, and an ultrasonic image forming unit 66 that forms a two-dimensional ultrasonic image, a three-dimensional ultrasonic image, or various Doppler images from an echo signal.
The ultrasonic pulse generation unit 64 and the conversion unit 65 may be any ultrasonic transmitter / receiver provided separately from the control unit 63, for example.

The display unit 61 is an LCD (Liquid Crystal Display) or a monitoring device, and is a display means for displaying an image formed by the ultrasonic image forming unit 66.
The operation unit 62 is an input means for inputting parameters and the like so that the operator can appropriately diagnose the object 9 to be measured, and a push button, a touch panel, or the like may be used.

As shown in FIG. 1, the ultrasonic probe 1 is electrically connected to the control unit 63 via a cable or the like, and transmits an ultrasonic signal toward a measurement object 9 which is a human body or an object. , The ultrasonic signal reflected as an echo from the measurement object 9 is received.
The ultrasonic diagnostic apparatus 10 can visualize and diagnose the inside of the measurement object 9 by transmitting and receiving such an ultrasonic signal.
Alternatively, as shown in FIG. 3, the diagnosis may be made using the terminal 50 and the ultrasonic probe 1 connected to the terminal 50 by a cable.

As shown in FIG. 4, the ultrasonic probe 1 includes a support portion 3 which is a support substrate, a PMUT chip 2 which is an ultrasonic transducer formed on the support portion 3, a flexible substrate 4, and wiring. It has a 5, a connector 7, and an acoustic lens 8.
The PMUT chip 2 is connected to the flexible substrate 4 via the wiring 5, and is connected from the connector 7 to the control unit 63 via the circuit board.

The support portion 3 has a function as a backing plate for holding the PMUT chip 2.
The acoustic lens 8 is an acoustic lens made of silicon resin for focusing the ultrasonic waves transmitted from the PMUT chip 2 on the measurement position of the measurement object 9.
The acoustic lens 8 has a so-called dome shape in which the central portion is thicker than the peripheral portion, and by abutting and adhering to the measurement object 9, the ultrasonic wave is simulated by the difference in thickness between the central portion and the peripheral portion. It has a function of refracting and focusing. The acoustic lens 8 may have a function of focusing ultrasonic waves in at least one direction, and does not necessarily have to focus ultrasonic waves at one point.
The acoustic lens 8 and the PMUT chip 2 are bonded to each other by an adhesive 6.

As shown in FIG. 5, the PMUT chip 2 has a silicon substrate 11, an oxide film 13 formed on the silicon substrate 11, a Si layer 14, and an oxide film 15.
The oxide film 13, the Si layer 14, and the oxide film 15 operate as a diaphragm 16 when a voltage is applied to the piezoelectric element 20 as described later.
In the present embodiment, a plurality of so-called dome-shaped piezoelectric elements 20 having a central portion thicker than the peripheral portion are arranged on the upper portion of the diaphragm 16.
A gap 30 is formed on the opposite side of the piezoelectric element 20 across the diaphragm 16, that is, on the lower side in FIG. 5, as shown by the broken line.
In the description after FIG. 5, the direction perpendicular to the surface of the silicon substrate 11 is the Z-axis, the arrangement direction of the piezoelectric elements 20 formed on the silicon substrate 11 is the X-axis, and the direction perpendicular to the X-axis and the Z-axis. Will be described as the Y-axis.

When viewed from the upper side of the Z-axis, the PMUT chip 2 has a plurality of piezoelectric elements 20 arranged in an array, and the respective piezoelectric elements 20 are electrically connected by a signal line 29, as shown in FIG. The signal line 29 is connected to the wiring 5 at the end of the PMUT chip 2 as shown in FIG. 3 and already described.
The signal line 29 extends in the X direction and is connected to the upper electrode 23, which will be described later.
As shown in FIG. 6, a GND line 28, which is a ground line serving as a reference potential, is provided around the PMUT chip 2 in which the piezoelectric elements 20 are arranged, and is connected to a lower electrode 21 described later.

The gap portion 30 is a cylindrical opening opened in the silicon substrate 11, and as shown in the cross-sectional view in FIG. 7, the upper bottom surface of the gap portion 30 on the + Z side is an oxide film 13.
Note that FIG. 7 shows a cross-sectional view when the piezoelectric element 20 and the substrate 11 are cut on a plane including the central axis and the X axis of one piezoelectric element 20, and the width of the gap 30 in the X direction: Cw.

Further, in the present embodiment, as is clear from FIG. 7, the piezoelectric element 20 is composed of a lower electrode 21 which is a first electrode formed on the vibrating plate 16, a dome-shaped piezoelectric body 22, and a piezoelectric body 22. It has an upper electrode 23, which is a second electrode formed on the upper surface.
Further, the piezoelectric element 20 has an insulating layer 24 provided above the upper electrode 23, a signal line 29, and a protective layer 25 for protecting the signal line 29.
In the present embodiment, the piezoelectric body 22 has a dome-shaped structure, but the structure is not limited to this.

The lower electrode 21 is a platinum (Pt) layer in the present embodiment, but is not limited to such a material, and a conductive metal material or the like can be used.
The upper electrode 23 is also a platinum (Pt) layer, and is not limited to such a material, and a conductive metal material or the like can be used. The upper electrode 23 and the lower electrode 21 are preferably made of the same material, but different materials may be used.
The piezoelectric body 22 is a piezoelectric material made of lead zirconate titanate (PZT) in the present embodiment, and has a dome shape in which the central portion 22a is formed thicker than the peripheral portion 22b.
The piezoelectric body 22 undergoes mechanical deformation by applying a drive voltage between the upper electrode 23 and the lower electrode 21, and vibrates at a predetermined frequency due to periodic fluctuations in the drive voltage, thereby causing contact vibration. The plate 16 is vibrated to generate ultrasonic waves.
Further, when the ultrasonic wave vibrates the piezoelectric body 22, the piezoelectric body 22 is polarized to generate a potential difference between the upper electrode 23 and the lower electrode 21, and also functions as a detection means for detecting the vibration as an electric signal.
As described above, in the present embodiment, the PMUT chip 2 functions as an electromechanical conversion element that periodically expands and contracts the piezoelectric body 22 by a potential difference between the upper electrode 23 and the lower electrode 21, that is, an electric signal to generate vibration. Have. In particular, the present embodiment has a function as an ultrasonic transducer that oscillates a sound wave in an ultrasonic region by such vibration.

The insulating layer 24 is an insulating film for preventing a short circuit between the upper electrode 23 and the lower electrode 21 and a short circuit between the signal line 29 and the lower electrode 21.

As a first embodiment of the present invention, an example of a method for manufacturing a PMUT chip 2 having such a piezoelectric element 20 is shown.
As shown in FIG. 8A, an oxide film 13 is formed on a general silicon substrate 11 as an oxide film called a BOX layer for SOI with a thickness of about 50 nm to 1000 nm.
Next, the Si layer 14 which is an SOI active layer is formed in an amount of 0 to 5 μm, and an oxide film 15 which functions as an insulating layer is formed on the layer 14 in an amount of about 50 nm to 1000 nm. Such a configuration is a substrate forming step. Since the method for manufacturing these SOI wafers may be general, it is omitted in this description.

Next, as shown in FIG. 8B, the lower electrode 21 which is the first electrode is formed.
_{At this time, a titanium dioxide (TiO 2} ) layer having a size of about 50 to 200 nm may be formed as an adhesion layer between the lower electrode 21 and the oxide film 15 as a base.
Examples of the method for producing the titanium dioxide (TiO ₂ ) layer include a method in which titanium is formed into a film of 30 to 200 nm by a sputtering method and then oxidized by RTA (Rapid Thermal Anneal) in an oxygen atmosphere.

The lower electrode 21 is a platinum (Pt) layer as described above, and for example, platinum (Pt) is formed into a film of 50 to 500 nm by a sputtering method. Such a configuration is a first electrode forming step.
As shown in FIG. 8B, lead acetate, zirconium alkoxide compound, and titanium alkoxide compound are used as starting materials for the lead zirconate titanate (PZT) film on the upper part of the lower electrode 21 by the so-called CSD (Chemical Solution Deposition) method. The precursor solution is spin-coated and then dried, thermally decomposed, and crystallized to form a film.
At this time, the liquid discharge head 100 selectively applies the CSD droplet 38 to the piezoelectric layer 36a, which is in the process of forming the piezoelectric body 22.
The liquid discharge head 100 is an inkjet head capable of applying the CSD droplet 38 to an arbitrary position on the silicon substrate 11 by moving the silicon substrate 11 or the liquid discharge head 100.
At this time, a surface treatment is performed so that the surface of the portion forming the piezoelectric layer 36b becomes hydrophilic and the outer peripheral surface of the portion forming the piezoelectric layer 36b becomes water repellent by a known method.
When the CSD droplet 38 is applied to the substrate in such a surface state, the CSD droplet 38 is hydrophilic even when the position where the CSD droplet 38 lands is slightly varied due to a minute position error of the liquid discharge head 100. The CSD droplet 38 is selectively applied only to the portion having the property, and the CSD droplet 38 is not applied to the portion having the water repellency.
After the CSD droplet 38 is applied to the desired position in this way, the partially applied CSD droplet 38 is dried, thermally decomposed, and crystallized.
If the film thickness formed by one coating is increased, there is a concern that cracks are likely to occur. Therefore, in the present embodiment, the CSD droplet 38 coated by the liquid discharge head 100 is a film after crystallization. The thickness is adjusted to be within about 100 nm.
After the crystallization is completed, the liquid discharge head 100 repeatedly applies the CSD droplet 38 again until a piezoelectric body 22 having a desired thickness is obtained. Such a step is a piezoelectric body forming step of forming a piezoelectric body.

In the present embodiment, the piezoelectric body 22 having a size of 1 μm to 4 μm is formed by repeating the above coating step 10 to 40 times.
At this time, the viscosity and drying rate of the CSD droplet 38 can be controlled. For example, increasing the viscosity increases the curvature of the surface of the piezoelectric body 22 after the film formation.
By controlling the physical properties of the CSD droplet 38 used as the coating material in this way, it is possible to control the thickness of the central portion and the peripheral portion of the piezoelectric body 22 to be formed into a film.

Further, as shown in FIG. 8C, a platinum upper electrode 23 having a thickness of 50 to 300 nm is formed on the + Z side surface of the formed piezoelectric body 22 by a sputtering method or the like. Such a step is a second electrode forming step of forming the second electrode.
At this time, the upper electrode 23 is formed along the upper surface side of the dome-shaped piezoelectric body 22, and it is desirable that the diameter of the upper electrode 23 is smaller than the outer diameter of the piezoelectric body 22, and in particular, the dome-shaped piezoelectric body 22. In this case, the diameter of the upper electrode 23 can be made smaller than the outer shape of the piezoelectric body 22 to prevent a short circuit between the upper electrode 23 and the lower electrode 21.
The upper electrode 23 is patterned in a predetermined pattern by photolithography etching.

Next, as shown in FIG. 8D, an insulating layer 24 is formed as an insulating film on the upper electrode 23. Silicon In this embodiment, the insulating layer 24, which by a plasma CVD method, monosilane _(SiH 4), nitrous oxide _(N 2 O) gas was formed by forming a film of 0.4~1.0μm as a raw material It is an oxide film.
A contact hole 41 is opened in the insulating layer 24 by photolithography etching, a wiring material of Al—Cu (1 μm) / Ti (50 nm) is formed by a sputtering method, and a signal line 29 is formed through patterning by photolithography etching. Will be done.
Further, a silicon nitride film (Si ₃ N ₄ ) is formed as the protective layer 25, and only the electrode terminal portion connected to the wiring 5 is opened.
In the present embodiment, the protective layer 25 is a _{silicon nitride film (Si 3} N ₄ ) of about 0.5 μm to 1.5 μm using _{monosilane (SiH 4} ) and nitrous ammonia (NH _{3) gas as raw materials by a plasma CVD method.} Was formed.
The opening of the electrode terminal portion was performed by photolithography etching.

As shown in FIG. 8D, the gap portion 30 is formed on the side of the silicon substrate 11 opposite to the piezoelectric body 22, that is, on the back surface side of the silicon substrate 11.
Specifically, after the thickness of the silicon substrate 11 is adjusted to about 20 μm to 200 μm by back grinding and polishing, the surface (= surface, + Z side surface) on which the piezoelectric element 20 of the silicon substrate 11 is formed is attached to the support substrate. ..
A resist mask having a desired pattern is formed on the back surface side (−Z side surface) of the silicon substrate 11 with a photolithography, and the silicon substrate 11 is etched. The etching process uses a silicon deep moat etcher and can be easily performed by a so-called Bosch process (a process in which etching by _{SF 6} plasma and formation of a side wall protective film by _{C 4} F _{8 plasma are alternately repeated).}
At this time, the portion of the silicon substrate 11 on which the resist mask is not formed is dug down until the oxide film 13 is exposed by etching to form the void portion 30. Such a step is a gap forming step of forming the gap 30 on the side opposite to the piezoelectric body 22 of the silicon substrate 11.
Finally, the support wafer is peeled off, dicing and the like are performed to complete the wafer generation process of the PMUT chip 2.

By the way, when the PMUT chip 2 generated through such a manufacturing process is used as the ultrasonic transducer of the ultrasonic probe 1, the sound pressure is secured, and the size, thickness, and frequency are increased. Both were required.
However, it is known that the miniaturization and the increase in frequency of the piezoelectric element generally lead to a decrease in sound pressure due to a decrease in the amplitude of the vibrating portion.
However, according to the research by the inventors, by appropriately controlling the size of the piezoelectric body 22, the size of the upper electrode 23, and the size of the gap portion 30, high frequency, high resolution, appropriate sound pressure and reception sensitivity are obtained. It has become clear that it is possible to secure.

This point will be described in more detail. In the cross-sectional view of FIG. 7 shown above, the width of the gap portion 30 when the cross section of the piezoelectric element 20 of the gap portion 30 and the silicon substrate 11 is cut is defined as Cw.
The "width of the gap portion 30" is, for example, the width of the gap portion 30 when the piezoelectric body 22 is viewed from the Z direction, which is the stacking direction, and the width of the gap portion 30 when viewed from the cross section including the central axis of the piezoelectric body 22. It may be in the X direction, the Y direction, or any other direction. In the present embodiment, since the gap portion 30 is a cylindrical space, the width of the gap portion 30: Cw is equal in the entire circumferential direction, and therefore, it is particularly described as a cavity diameter in FIG.
Further, since the piezoelectric body 22 has a circular dome shape when viewed from above, the diameter (PZT diameter) of the piezoelectric body 22 is set to the width of the piezoelectric body 22: Pw, the thickness of the central portion 22a of the piezoelectric body 22: Ptc, and so on. The thickness of the peripheral edge portion 22b of the piezoelectric body 22 is defined as the thickness of the piezoelectric body 22 at the end portion of the upper electrode 23: Pte. As described above, the "width of the piezoelectric body 22: Pw" is the width of the piezoelectric body 22 when viewed from the direction in which the piezoelectric elements 20 are laminated, and in FIG. 7, a cross-sectional view is used for simplification of the description. Described.
FIG. 9 shows an image when the ultrasonic transducer vibrates. The upper wiring and insulating film are omitted for the sake of simplicity.
By applying an electric potential to the upper electrode 23 and the lower electrode 21 of the piezoelectric body 22 at a high frequency, the piezoelectric body 22 expands and contracts in the horizontal direction, and as a result, the piezoelectric element 20 and the diaphragm 16 vibrate in the vertical direction. This vibration is output as ultrasonic waves. Since the silicon substrate 11 is fixed to the piezoelectric element 20, the piezoelectric element 20 vibrates at the inflection point 31.
When vibrating with the same energy, the area around the inflection point is soft, and the lighter the vibration, the larger the displacement, and as a result, the sound pressure and reception voltage increase. Therefore, it is preferable that there is no hard / heavy material such as the piezoelectric material 22 in the vicinity of the inflection point 31. FIG. 10 is an enlarged view of the vicinity of the inflection point of FIG. Assuming that the distance in the direction perpendicular to the vibration direction generated between the end of the gap 30 and the end of the piezoelectric 22 is the space 32, the space 32 uses Cw and Pw used in FIG. 7 (Cw). It can be represented by −Pw) / 2.
The space 32 is required to have a certain distance in consideration of the deviation of the pattern at the time of manufacturing the piezoelectric element. Further, assuming that the distance in the direction perpendicular to the vibration direction generated between the end portion of the piezoelectric body 22 and the end portion of the upper electrode 23 is the space 33, the space 33 can be represented by (Pw-Uw) / 2. can. Since the portion represented by the space 33 is only the piezoelectric body 22 and does not have the upper electrode 23, it does not affect the piezoelectric characteristics. Therefore, it is desirable that the space 33 is thin and narrow.

As shown in FIG. 11, as the width corresponding to the width of the gap 30: Cw increases, the frequency of ultrasonic waves generated by the resonance of the diaphragm 16 decreases.
Specifically, in order to cause resonance in the frequency range of 5 MHz to 20 MHz, the cavity diameter is about 30 μm to 100 μm.
In the present embodiment, the frequency is set to 10 MHz and the width of the gap 30 is within the range of Cw = 60 μm to 70 μm.

Next, the shape of the piezoelectric body 22 will be described.
In the present embodiment, the piezoelectric body 22 has a dome shape in which the central portion 22a is thicker than the peripheral portion 22b as described in FIG. 8B.
When the piezoelectric body 22 has a cylindrical shape, the bendability of the piezoelectric body 22 is uniform, but when the piezoelectric body 22 has a dome shape, the edge portion is thin, so that the peripheral portion 22b side is relatively displaced. It is easy, and as a result, the sound pressure and the reception sensitivity can be maintained higher in the dome shape.
FIG. 12 shows a diagram in which the shape of the piezoelectric body 22 and the sound pressure are relatively evaluated in this way. In FIG. 12, the sound pressure value on the vertical axis is a relative value, which is an arbitrary unit when a predetermined sound pressure is 1.
Further, FIG. 12 is a graph showing the magnitude of the sound pressure detected when ultrasonic waves are emitted from the PMUT chip 2 as an ultrasonic transducer.

As is clear from FIG. 12, when the thickness at the central portion 22a of the piezoelectric body 22 is Ptc, if the thickness of the piezoelectric body 22 is the same between the cylindrical shape and the dome shape, the dome shape has a higher sound pressure. Since it can be secured, it is preferable that the piezoelectric body 22 has a dome shape.
Further, when the piezoelectric body 22 has a cylindrical shape, the sound pressure decreases as the thickness: Ptc in the central portion 22a of the piezoelectric body 22 increases due to its weight, but when the piezoelectric body 22 has a dome shape, the central portion 22a of the piezoelectric body 22 has a shape. Thickness: The decrease in sound pressure can be suppressed even when Ptc is increased.

FIG. 13 shows the relationship between the PZT diameter and the transmitted sound pressure, and FIG. 14 shows the relationship between the PZT diameter and the received voltage.

Therefore, in the present embodiment, when the width of the gap portion 30 which is the Caviity diameter is Cw and the width of the piezoelectric body 22 corresponding to the diameter of the piezoelectric body 22 (PZT diameter) is Pw, the piezoelectric element 20 is based on the conditional equation (PZT diameter). Satisfy 1).

The conditional expression (1) is an expression that defines the size of the piezoelectric body 22 with respect to the size of the gap portion 30. If it is smaller than the lower limit of the conditional expression (1), the piezoelectric body 22 becomes too small with respect to the gap portion 30, so that the diaphragm 16 does not vibrate sufficiently, resulting in a decrease in sound pressure.
On the other hand, if it becomes larger than the upper limit of the conditional expression (1), the piezoelectric body 22 becomes too large with respect to the gap portion 30, and as shown in FIGS. 9 and 10, the piezoelectric body 22 is heavy and hard, so that the diaphragm 16 does not vibrate sufficiently, causing a decrease in sound pressure.
As is clear from FIG. 14, for the same reason as for the reception sensitivity, the width of the piezoelectric body 22 and the width of the gap 30 are set within the range of the conditional expression (1) so that the relative sound pressure value is 0.5 or more. Can be secured. Further, it is more preferable to set 0.7 ≦ Pw / Cw ≦ 0.9 within the range of (1) because the relative sound pressure value can be further increased.

Further, when the width of the upper electrode 23 in FIG. 7 is Uw, the piezoelectric element 20 satisfies the conditional expression (2).

The conditional expression (2) is an expression that defines the size of the upper electrode 23 with respect to the piezoelectric body 22. The relationship between the conditional expression (2) according to FIG. 15 and the relative sound pressure value is illustrated.
As is clear from FIG. 15, as Uw becomes larger (as Uw becomes larger than Pw), the piezoelectric effect increases and the sound pressure is improved.
However, as shown in FIG. 16, the upper electrode and the received voltage become smaller as Uw becomes larger than Pw. The relationship between the width of the upper electrode 23 and the capacitance of the piezoelectric element is shown in FIG. As shown in FIG. 17, as the upper electrode 23 becomes larger, the piezoelectric element capacitance of the piezoelectric body 22 sandwiched between the upper and lower electrodes increases.
When the capacitance of such a piezoelectric element becomes large, an unintended C component is placed on the circuit, which causes a decrease in responsiveness and a decrease in voltage in a high frequency band.
In this embodiment, in particular, a plurality of piezoelectric elements 20 are arranged in the PMUT chip 2, each of which has an upper electrode 23 and a lower electrode 21, and the lower electrode 21 is shared in the array direction. That is, since the piezoelectric element capacitance becomes a parallel combined capacitance, the influence of signal delay and voltage drop is large, and it is preferable to reduce the piezoelectric element capacitance per piezoelectric element 20 as much as possible.
Therefore, it is important to design the optimum value in combination with the results of FIGS. 15, 16 and 17.
Therefore, in the present embodiment, the piezoelectric element 20 satisfies the conditional equation (2) to reduce the capacitance of the piezoelectric element, and the width of the piezoelectric body 22: Pw and the width of the upper electrode 23: Uw are set to the conditional equation (2). ), While ensuring a relative sound pressure value of 0.5 or more, the capacitance of the piezoelectric element can be reduced while securing the reception voltage, and a decrease in responsiveness to high frequencies can be suppressed.
Further, it is more preferable to set 0.7 ≦ Uw / Pw ≦ 0.8 within the range of the conditional expression (2) because both positive response and sound pressure can be achieved at the same time.

Further, when the thickness of the piezoelectric body 22 at the end of the upper electrode 23 is Pte and the thickness of the central portion 22a of the piezoelectric body 22 is Ptc, the piezoelectric element 20 satisfies the conditional expression (3).

Generally, the dome-shaped piezoelectric body shape is ^{approximated by a mathematical formula such as Y = -ax 2} + b (see, for example, Patent Document 6 and the like). FIG. 18 shows the relationship between the film thickness of the piezoelectric body 22 and the breakdown pressure resistance.
The thickness of the piezoelectric body 22 at the end of the upper electrode 23: Pte is a typical value of the thickness of the peripheral edge 22b, and if the Pte is thin, dielectric breakdown between the upper electrode 23 and the lower electrode 21 is broken. And if Pte = 0, it will be short-circuited. On the contrary, when Pte is thick, the dome shape becomes close to Ptc, and the width of the upper electrode 23: Uw becomes small, so that the conditional expression (2) is not satisfied.
Further, FIG. 19 shows the relationship between the thickness of the piezoelectric body 22 and the capacitance of the piezoelectric element. As is clear from FIG. 19, the piezoelectric element capacity can be reduced by increasing the film thickness of the piezoelectric body 22.
Therefore, by satisfying the conditional expression (3), it is possible to secure the breakdown withstand voltage between the upper electrode 23 and the lower electrode 21, improve the sound pressure and the received voltage, and optimize the piezoelectric element capacitance. Further, in the present embodiment, as described above, the width of the upper electrode 23: Uw and the width of the piezoelectric body 22: Pw are regulated within the range of the conditional expression (2). Therefore, in the present embodiment, in addition to the conditional expression (3), it is more preferable that the thickness of the piezoelectric body 22 at the end of the upper electrode 23 is within the range of Pte ≦ 3 μm.
Further, setting the range of Pte within the range of (3) to 1 μm ≦ Pte ≦ 2 μm is more preferable because durability against dielectric breakdown and improvement of sound pressure due to the dome shape can be achieved at the same time.

In the present embodiment, the conditional expression (4) is satisfied when the thickness at the central portion 22a of the piezoelectric body 22 is Ptc.

If it is larger than the upper limit of the conditional expression (4), the film thickness of the piezoelectric body 22 becomes too large, so that cracks and the like are likely to occur.
Further, if it is smaller than the lower limit of the conditional expression (4), as already described, the piezoelectric element capacity is increased and the withstand voltage is also lowered, which is not preferable.
Further, by setting the range of Ptc to the range of 1.5 μm ≦ Ptc ≦ 3 μm within the range of (4), durability by suppressing cracks and improvement of sound pressure due to the dome shape can be achieved at the same time, which is more preferable.

The optimization of the size of the typical portion of the piezoelectric element 20 in the present embodiment has been described above, but the size is given as an example of the embodiment and is not limited to the size. ..

For example, in the present embodiment, the piezoelectric body 22 has a dome shape that is axisymmetric with respect to the center, and satisfies the conditional expression (1) at any position, but as shown in FIG. 20 (a), the piezoelectric body 22 When the dome shape is elliptical when viewed from the + Z direction side, for example, the conditional equation (1) is used in both the AA'direction cross section and the BB'direction cross section orthogonal to the A direction. May be formed to satisfy. Further, as shown in FIG. 21, not only the piezoelectric body 22 but also the gap portion 30 may have an elliptical shape.
An example of a cross-sectional view of the piezoelectric body 22 is shown in FIGS. 20 (b) and 20 (c).

Further, as a second embodiment of the present invention, a configuration when the shape of the piezoelectric body 22'provided in the piezoelectric element 20 is changed from a dome shape to a cylindrical shape will be described.
As shown in FIG. 22, in the present embodiment, the piezoelectric body 22'is formed in a columnar shape having no difference in thickness between the central portion and the peripheral portion. The same configurations as those described in the first embodiment are designated by the same reference numerals, and the description thereof will be omitted as appropriate. In addition, the description of the parts that do not make a big difference due to the cylindrical shape of the insulating layer 24, the protective layer 25, the signal line 29, etc. is omitted.

Regarding such a piezoelectric body 22', the width of the gap 30 when the cross section of the piezoelectric element 20 and the silicon substrate 11 is cut in a plane including the center of the piezoelectric element 20: Cw, the width of the piezoelectric body 22' : Pw is defined to show its cross section in FIG. 23, respectively.
Specifically, the diameter of the cylindrical piezoelectric body 22'when viewed from the + Z direction is the width of the piezoelectric body 22': Pw, and similarly, the diameter of the gap portion 30 is the width of the gap portion 30: Cw.
Further, the height of the piezoelectric body 22'in the + Z direction is the thickness of the piezoelectric body 22': Pt, and as a matter of course, in this embodiment, Pt = Pte = Ptc.

Further, the upper electrode 23'is also a cylindrical platinum (Pt) electrode formed on the upper portion of the piezoelectric body 22'.
At this time, assuming that the width of the upper electrode 23'in the cross section of the piezoelectric element 20 is Uw, the width of the upper electrode 23'is defined as Uw as shown in FIG.

By the way, also in such a second embodiment, the PMUT chip 2 satisfies the conditional equation (1) and the conditional equation (2) when the width of the upper electrode in the cross section of the piezoelectric element 20 is Uw.
With such a configuration, high frequency, high resolution, appropriate sound pressure, and reception sensitivity can be ensured even in the embodiments shown in FIGS. 22 and 23.

In the case of forming the cylindrical piezoelectric body 22'with such a configuration, regardless of the inkjet method, a piezoelectric body 22' of 1 to 4 μm is formed by spin-coating with a sputtering, CVD or sol-gel solution, and photolithography is performed. It can be manufactured by patterning using etching.

In the first and second embodiments described above, the gap portion 30 is formed from the back surface side after the piezoelectric element 20 is formed on the silicon substrate 11 as shown in FIG. 8E as an example. However, as shown in FIG. 24, in the step of forming the SOI on the silicon substrate 11, a manufacturing method in which silicon is etched from the surface of the substrate by about 1 to 5 μm in advance, or a so-called sacrificial layer etching, in which the gap portion 30 is formed. A manufacturing method may be adopted in which a hole 34 serving as an opening is formed in a part of a portion constituting the wall surface, and the silicon substrate 11 is dry-etched from the hole 34.
Further, as for the shape of the gap portion 30, in addition to the cylindrical shape, various deformation examples may be taken according to the shape of the piezoelectric element 20.

Although the embodiments according to the present invention have been described above, the present invention is not limited to the above-described embodiments as they are, and at the implementation stage, the components can be modified and embodied within a range that does not deviate from the gist thereof. .. In addition, various inventions can be formed by an appropriate combination of the plurality of components disclosed in the above-described embodiment. For example, some components may be removed from all the components shown in the embodiments. Further, different embodiments and modifications may be combined as appropriate.

1 Ultrasonic probe 2 Ultrasonic transducer, electromechanical conversion element (PMUT chip)
3 Support part 8 Acoustic lens 9 Measurement object 10 Ultrasonic diagnostic device 11 Substrate part (silicon substrate)
20 Piezoelectric element 21 First electrode (lower electrode)
22 Piezoelectric body 22a Central part of piezoelectric body 22b Peripheral part of piezoelectric body 23 Second electrode (upper electrode)
30 Void portion 31 Inflection point 32 Space between the void portion and the end of the piezoelectric body (Cw-Pw) / 2
33 Space between the piezoelectric body and the upper electrode (Pw-Uw) / 2
61 Display unit 62 Operation unit 63 Control unit Cw Void width Pw Piezoelectric width Uw Upper electrode width Pte Piezoelectric thickness at the periphery Ptc Piezoelectric thickness at the center

Japanese Unexamined Patent Publication No. 2019-071361 Japanese Patent No. 6586705 Japanese Patent No. 5345917 Japanese Patent No. 5499479 Japanese Patent No. 5974486 Japanese Patent No. 6182968

Claims (11)

The substrate, the first electrode, the piezoelectric body, and the second electrode, which are sequentially laminated,
A gap formed in the substrate on the side opposite to the piezoelectric body across the first electrode, and
With
The width of the gap portion as seen from the direction of sequential stacking: Cw and the width of the piezoelectric body: Pw satisfy the conditional expression (1): 0.65 ≦ Pw / Cw ≦ 0.95. Electromechanical conversion element. The electromechanical conversion element according to claim 1.
When the width of the second electrode as seen from the stacking direction: Uw,
Conditional expression (2): 0.6 ≤ Uw / Pw ≤ 0.9
An electromechanical conversion element characterized by satisfying. The electromechanical conversion element according to claim 1 or 2.
The piezoelectric body is an electromechanical conversion element characterized in that the central portion is thicker than the peripheral portion in the laminated direction. The electromechanical conversion element according to claim 3.
The thickness of the piezoelectric body at the end of the second electrode: Pte.
When the thickness of the piezoelectric body at the center is Ptc,
Conditional expression (3): 0.2 ≤ Pte / Ptc ≤ 1.0
An electromechanical conversion element characterized by satisfying. The electromechanical conversion element according to claim 3 or 4.
Thickness of the piezoelectric material at the center: Ptc
Conditional expression (4): 1 μm ≤ Ptc ≤ 4 μm
An electromechanical conversion element characterized by satisfying. The electromechanical conversion element according to any one of claims 1 to 5.
An electrical conversion element characterized in that the shapes of the piezoelectric body and the gap portion seen from the stacking direction are parallel to the stacking direction and axially symmetric with respect to a central axis passing through the center of the piezoelectric body. The electromechanical conversion element according to any one of claims 1 to 6.
An electromechanical conversion element characterized in that the piezoelectric body and the gap portion are arranged in a plurality of arrays. An ultrasonic transducer using the electromechanical conversion element according to any one of claims 1 to 7. The ultrasonic transducer according to claim 8, an acoustic lens, and a support portion that supports the ultrasonic transducer and the acoustic lens.
By applying a voltage to the electromechanical conversion element to vibrate the bottom surface of the gap portion, ultrasonic waves are oscillated toward the object to be measured, and the vibration of the ultrasonic waves reflected by the object to be measured is detected. Ultrasonic probe featuring. The ultrasonic probe according to claim 9,
An ultrasonic diagnostic apparatus having a display unit for displaying the shape of the object to be measured. The substrate, the first electrode, the piezoelectric body, and the second electrode, which are sequentially laminated,
A gap formed in the substrate on the side opposite to the piezoelectric body across the first electrode, and
It is a manufacturing method of an electromechanical conversion element provided with
The first electrode forming step of forming the first electrode on the substrate, and
A piezoelectric body forming step of applying the piezoelectric body onto the first electrode by an inkjet method,
A second electrode forming step of forming a second electrode on a part of the upper surface of the piezoelectric body,
A gap forming step for forming a gap in the substrate,
Have,
An electric machine characterized in that the width of the void portion: Cw and the width of the piezoelectric body: Pw as viewed from the stacking direction satisfy the conditional expression (1): 0.65 ≦ Pw / Cw ≦ 0.95. Method of manufacturing a conversion element.

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