Electromechanical conversion element, ultrasonic transducer, ultrasonic probe, ultrasonic diagnostic device

Abstract

To provide an ultrasonic wave device structure with high reliability, in which both of an ultrawide waveband and transmission and reception efficiency are improved.SOLUTION: An electromechanical conversion element 18 comprises: a substrate 11, a first electrode 21, a piezoelectric substance 22, and a second electrode 23 which are sequentially laminated on a lamination direction; an insulation layer 24 that covers at least parts of the first electrode 21, the piezoelectric substance 22, and the second electrode 23; a protective layer 25 that covers the insulation layer 24; a gap part 30 that is formed on the substrate 11 which is opposite to the piezoelectric substance 22 across the first electrode 21; and a lamination film 33 that includes the first electrode 21, the piezoelectric substance 22, the second electrode 23, the insulation film 24, and the protective layer 25. A neutral surface of a strain of the lamination film 33 is positioned at the side opposite to the gap part 30 from an intermediate position B in the lamination direction of a cross section of the lamination film 33 at a center of a direction orthogonal to the lamination direction of the piezoelectric substance 22.SELECTED DRAWING: Figure 7

Description

[Technical field]

[0001] The present invention relates to an electromechanical conversion element, an ultrasonic transducer, an ultrasonic probe, and an ultrasonic diagnostic device. [Background technology]

[0002] Ultrasonic transducers, which transmit and receive ultrasonic waves by vibrating a thin film, are used in medical diagnosis, industrial, automotive, and marine inspection and measurement devices. Medical ultrasonic diagnostic devices are particularly popular because they cause no damage to living organisms and can easily observe internal tissues in real time. Conventionally, the electromechanical conversion elements used in such ultrasonic transducers have been diced from ceramic lead zirconate titanate (PZT), known as bulk. However, in recent years, technologies using semiconductor elements, known as Piezoelectric Micro-Machined Ultrasonic Transducers (PMUT) and Capacitive Micro-machined Ultrasonic Transducers (CMUT), have become known.

[0003] The above-mentioned technology can achieve high resolution and high frequency through microfabrication using semiconductor technology, and has the potential to become a mainstream technology in the future. In particular, CMUT has been put to practical use in highly functional and high-performance probes used in medical diagnostic equipment. CMUT can achieve acoustic characteristics approximating those of living tissue by making the diaphragm thin, soft, and light, making it possible to achieve an ultra-wideband without using an acoustic matching layer and provide high-quality diagnostic images. On the other hand, in order to improve the transmission and reception efficiency of PMUTs, technologies have been proposed to improve transmission and reception characteristics by examining the relationship between the thickness of the layer containing the piezoelectric material and the thickness of the diaphragm, the bending rigidity of the diaphragm and piezoelectric element, etc.

[0004] In the ultrasonic transducer described above, the CMUT was able to achieve an ultra-wide bandwidth, but the PMUT uses a piezoelectric film and differs from the CMUT in terms of vibration structure, materials used, etc., so there is room for improvement in achieving an ultra-wide bandwidth. This will be explained in detail below. In ultrasonic transducers, it is necessary to improve the receiving efficiency as well as the transmission efficiency, and the vibration structure is being studied from this viewpoint. In this case, the structure of the vibration plate capable of improving the displacement efficiency becomes important, and therefore it is necessary to improve the amount of vibration plate displacement caused by driving the piezoelectric element. As an example of such a configuration, for example, a piezoelectric body is formed on the vibration plate, and the neutral plane of stress is arranged so as to be located at the boundary between the piezoelectric body and the vibration plate or near the lower electrode on the vibration plate side of the piezoelectric body (for example, see "Patent Document 1" and "Patent Document 2"). Summary of the Invention [Problem to be solved by the invention]

[0005] On the other hand, in PMUT, multiple piezoelectric elements are often arranged in an array and driven simultaneously, in which case it is necessary to connect the upper electrodes of each piezoelectric element to be driven simultaneously with signal lines and wiring. The signal lines and wiring used in this case are made of aluminum or other materials, and a silicon nitride film is formed on the wiring as a protective layer. The silicon nitride film used here is a highly reliable protective layer suitable for mass production, and has a large Young's modulus of about 210 to 290 GPa. In a PMUT with densely arranged piezoelectric bodies, the entire surface of the vibration structure is covered with a silicon nitride film, and wiring such as aluminum and insulating films are also arranged above the piezoelectric bodies. Therefore, if the thickness of the vibration plate is increased in order to set the position of the neutral plane of stress near the boundary between the piezoelectric body and the vibration plate, the rigidity of the vibration structure will increase if a silicon nitride film with a large Young's modulus is used as described above.

[0006] On the other hand, in order to realize an ultra-wideband, it is necessary to make the vibration structure thin, soft, and light. As described above, in a structure in which a piezoelectric body is formed on a vibration plate and the neutral plane of stress is located at the boundary between the piezoelectric body and the vibration plate or in the vicinity of the lower electrode on the vibration plate side of the piezoelectric body, it is possible to improve the transmission and reception characteristics. However, the rigidity of the vibration structure increases, which is contrary to the thin, soft, and light vibration structure required to realize an ultra-wideband, and it has been difficult to provide an ultrasonic device structure that is highly reliable by improving both the ultra-wideband and the transmission and reception efficiency. An object of the present invention is to solve the above-mentioned problems and to provide a highly reliable ultrasonic device structure that achieves both an ultra-wideband and improved transmission and reception efficiency. [Means for solving the problem]

[0007] The invention described in claim 1 provides a piezoelectric element comprising a substrate, a first electrode, a piezoelectric body, a second electrode, an insulating layer covering at least a portion of the first electrode, the piezoelectric body, and the second electrode, a protective layer covering the insulating layer, and a gap portion formed in the substrate on the opposite side of the first electrode to the piezoelectric body, the first electrode being sandwiched between the piezoelectric body and the second electrode, the insulating layer covering the insulating layer, and a protective layer covering the insulating layer. Insulation layer and the protective layer, and the neutral plane of the distortion of the laminated film is located on the opposite side to the gap portion from the midpoint in the stacking direction of the cross section of the laminated film at the center of the direction perpendicular to the stacking direction of the piezoelectric body. Effect of the Invention

[0008] According to the present invention, it is possible to ensure rigidity on the protective layer side and configure the vibration structure to be thin, soft, and light, thereby providing a highly reliable ultrasonic device structure that achieves both ultra-wideband and improved transmission and reception efficiency. [Brief description of the drawings]

[0009] [Figure 1] 1 is a schematic diagram illustrating an ultrasonic diagnostic apparatus according to an embodiment of the present invention. [Diagram 2]2 is a schematic diagram illustrating a control unit of the ultrasound diagnostic apparatus according to the embodiment of the present invention. FIG. [Diagram 3] FIG. 2 is a schematic diagram showing another configuration of an ultrasonic diagnostic apparatus according to an embodiment of the present invention. [Figure 4] 1 is a schematic diagram illustrating an ultrasonic probe used in one embodiment of the present invention. [Diagram 5] FIG. 2 is a schematic perspective view illustrating an ultrasonic transducer used in one embodiment of the present invention. [Figure 6] FIG. 2 is a schematic plan view illustrating an ultrasonic transducer used in one embodiment of the present invention. [Figure 7] FIG. 2A is a schematic cross-sectional view illustrating a piezoelectric element used in one embodiment of the present invention; FIG. 2B is a schematic cross-sectional view taken along the line AA in FIG. [Figure 8] 3A to 3C are schematic diagrams illustrating an example of a manufacturing process for an ultrasonic transducer used in one embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] 1 is a schematic diagram showing an ultrasonic diagnostic apparatus according to an embodiment of the present invention. In the figure, an ultrasonic diagnostic apparatus 10 includes an ultrasonic probe 1 that transmits ultrasonic waves toward a measurement object 9 and detects the vibration of the ultrasonic waves reflected by the measurement object 9, a display unit 61 that visualizes and displays a signal from the ultrasonic probe 1, an operation unit 62 operated by an operator, and a control unit 63 as a terminal that controls the ultrasonic probe 1.

[0011] As shown in FIG. 2, the control unit 63 has an ultrasonic pulse generating unit 64 that generates a pulsed electrical signal for generating an ultrasonic signal, a conversion unit 65 that converts the echo signal received from the ultrasonic probe 1 into an electrical 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 the echo signal. The ultrasonic pulse generating unit 64 and the converting unit 65 may be, for example, any ultrasonic transceiver provided separately from the control unit 63 .

[0012] The display unit 61 is an LCD (Liquid Crystal Display) or a monitor 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 unit for inputting parameters so that an operator can perform an appropriate diagnosis on the measurement object 9, and uses push buttons, a touch panel, or the like.

[0013] As shown in FIG. 1, the ultrasonic probe 1 is electrically connected to a control unit 63 via a cable or the like, transmits an ultrasonic signal toward a measurement object 9, which may be a human body or an object, and receives the ultrasonic signal reflected as an echo from the measurement object 9. The ultrasonic diagnostic device 10 is capable of visualizing and diagnosing the inside of the measurement object 9 by transmitting and receiving such ultrasonic signals. The ultrasonic diagnostic device may be one that performs diagnosis using a terminal 50 and an ultrasonic probe 1 connected to the terminal 50 by a cable, as shown in FIG.

[0014] As shown in FIG. 4, the ultrasonic probe 1 has a support portion 3 which is a support substrate, a PMUT chip 2 as an ultrasonic transducer formed on the upper portion of the support portion 3, a flexible substrate 4, wiring 5, a connector 7, an acoustic lens 8, etc. The PMUT chip 2 is connected to a flexible substrate 4 via wiring 5, and is connected to a control unit 63 from a connector 7 via a circuit board. The support portion 3 functions as a backing plate for holding the PMUT chip 2 .

[0015] The acoustic lens 8 is an acoustic lens made of silicone resin for focusing the ultrasonic waves emitted from the PMUT chip 2 onto the measurement position of the measurement object 9 . The acoustic lens 8 has a so-called dome shape with the center being thicker than the periphery, and is in close contact with the measurement object 9 by deforming, and has the function of refracting and converging ultrasonic waves in a pseudo manner due to the difference in ultrasonic propagation speed caused by the difference in thickness between the center and the periphery. Note that the acoustic lens 8 only needs to have the function of converging ultrasonic waves in at least one direction, and does not necessarily have to converge ultrasonic waves to one point. The acoustic lens 8 and the PMUT chip 2 are bonded together by an adhesive 6 as a matching layer.

[0016] As shown in Fig. 5, the PMUT chip 2 has a silicon substrate 11 as a substrate, a diaphragm 16 formed on the silicon substrate 11, and a lower electrode 21 as a first electrode formed on the diaphragm 16. In this embodiment, a plurality of cylindrical piezoelectric elements 20 are arranged on the upper part of the diaphragm 16. On the opposite side of the diaphragm 16 to the piezoelectric elements 20, i.e., on the lower side in Fig. 5, a void portion 30 is formed in the silicon substrate 11 as indicated by a dashed line. In the explanation from FIG. 5 onwards, the direction perpendicular to the surface of silicon substrate 11, i.e., the up-down direction in FIG. 5, will be referred to as the Z-axis direction, which is the stacking direction in which substrate 11, vibration plate 16, lower electrode 21, etc. are stacked in sequence; the arrangement direction of piezoelectric elements 20 formed on silicon substrate 11, i.e., the left-right direction in FIG. 5, will be referred to as the X-axis direction; and the directions perpendicular to the X-axis and Z-axis directions, i.e., the depth direction in FIG. 5, will be referred to as the Y-axis direction.

[0017] When viewed from above in the Z-axis direction, the PMUT chip 2 has a plurality of piezoelectric elements 20 arranged in an array as shown in Fig. 6, and each piezoelectric element 20 is electrically connected to a signal line 29. As shown in Fig. 6, the signal line 29 is connected to the wiring 5 at the end of the PMUT chip 2, and is extended in the X-axis direction to be connected to an upper electrode 23 described later. 6, a GND line 28, which is a ground line serving as a reference potential, is provided around the arrangement of the piezoelectric elements 20 of the PMUT chip 2. The GND line 28 is connected to the lower electrode 21.

[0018] Figure 7(a) shows a cross-sectional view of a piezoelectric element 20 and a silicon substrate 11 cut along a plane including the central axis of one piezoelectric element 20 in the Z-axis direction and the X-axis, while Figure 7(b) shows a partial cross-sectional view of the center of the piezoelectric element 20 in the Y-axis direction (or the X-axis direction), which is a direction perpendicular to the Z-axis direction of the piezoelectric element 20, when Figure 7(a) is cut along the AA section, which is a plane including the central axis of the piezoelectric element 20 in the Z-axis direction. The cavity 30 is a cylindrical opening formed on the silicon substrate 11, and the bottom surface of the cavity 30 on the upper side in the Z-axis direction is the vibration membrane 16. The size of the cavity 30 in the X-axis direction, which is a direction perpendicular to the Z-axis direction in which the layers constituting the piezoelectric element 20 are stacked, is configured to be Cw.

[0019] As is apparent from Figures 5, 6 and 7, the piezoelectric element 20 has a lower electrode 21 formed on the vibration plate 16, a cylindrical piezoelectric body 22, and an upper electrode 23 as a second electrode formed on the upper surface of the piezoelectric body 22. Furthermore, the piezoelectric element 20 has an insulating layer 24 provided on the upper electrode 23, a signal line 29, and a protective layer 25 for protecting the signal line 29. In this embodiment, a cylindrical shape is used as the piezoelectric body 22, but the shape of the piezoelectric body is not limited to this, and piezoelectric bodies of other shapes may be used.

[0020] Although a platinum (Pt) layer is mainly used as the lower electrode 21, the material of the lower electrode 21 is not limited to this, and it is possible to use a conductive metal material, a conductive oxide, a combination thereof, etc. In this embodiment, SrRuO3, LaNiO3, etc. are used as the oxide electrode layer, and Pt, Ti, Cr, Ta, Rh, Pd, etc. are used as the metal electrode layer. Although not shown, other layers such as an adhesive layer may be provided between the lower electrode 21 and the diaphragm 16. For example, a titanium layer, a titanium oxide layer, a titanium tungsten layer, an alumina layer, a zirconium oxide layer, or the like can be formed between the lower electrode 21 and the diaphragm 16. Such an adhesive layer is generally thin, about several tens of nm, and therefore makes little contribution to the rigidity of the laminated film 33 (including the insulating layer 24) constituting the piezoelectric element 20 shown in FIG. 7(b).

[0021] Although a platinum (Pt) layer is also mainly used for the upper electrode 23, the material of the upper electrode 23 is not limited to this, and a conductive metal material, a conductive oxide, a combination thereof, etc. can be used. Therefore, the materials that can be specifically used for the upper electrode 23 are not only similar to those for the lower electrode 21, but also have a wider selection than the lower electrode 21. In this embodiment, SrRuO3, LaNiO3, IrO2, RuO2, SrO, CaRuO3, etc. are used as the oxide electrode layer, and Pt, Ir, Ru, Ti, Cr, Ta, Rh, Pd, etc. are used as the metal electrode layer.

[0022] The piezoelectric body 22 is made of a lead zirconate titanate (PZT)-based piezoelectric material and has a cylindrical shape. Of course, the PZT-based composite oxide may contain other elements. When a drive voltage is applied between the upper electrode 23 and the lower electrode 21, the piezoelectric body 22 undergoes mechanical deformation, and the periodic fluctuation of the drive voltage causes vibrations of a predetermined frequency, which vibrates the contacting diaphragm 16 and generates ultrasonic waves. When the ultrasonic waves vibrate the piezoelectric body 22, the piezoelectric body 22 becomes polarized, generating a potential difference between the upper electrode 23 and the lower electrode 21, and the piezoelectric body 22 also functions as a detection means for detecting the vibrations as an electrical signal. Thus, in this embodiment, the PMUT chip 2 functions as an electromechanical conversion element that generates vibrations by periodically expanding and contracting the piezoelectric body 22 in response to the potential difference between the upper electrode 23 and the lower electrode 21, i.e., an electrical signal. In this embodiment, the PMUT chip 2 also functions as an ultrasonic transducer that generates sound waves in the ultrasonic range by the above-mentioned vibrations.

[0023] 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. A silicon oxide film is used as the insulating layer 24. An aluminum material or the like is used as the signal line 29, and a protective layer 25 is formed on the signal line 29. A silicon nitride film, which is a high-quality protective film with excellent mass productivity, is used as the protective layer 25. The silicon nitride film is suitable as a protective film for protecting the PMUT chip 2 from impurities, moisture, scratches, and the like.

[0024] In the PMUT chip 2 having the function of an electromechanical conversion element and an ultrasonic transducer shown in this embodiment, the neutral plane of strain in the cross section of the laminated film 33 having the piezoelectric body 22, the lower electrode 21, the upper electrode 23, the insulating layer 24, and the protective layer 25 is configured to be located on the opposite side to the gap 30 from the midpoint of the cross section of the laminated film 33 at the center of the piezoelectric body 22. Specifically, the neutral plane of strain is configured to be located on the opposite side to the gap 30, i.e., upward in FIG. 7(b), from the midpoint in the Z-axis direction of the cross section of the laminated film 33, as indicated by the symbol B in FIG. 7(b). Here, the neutral plane of strain used in this specification refers to the neutral plane of stress, and indicates a plane where the strain due to deformation becomes zero in the piezoelectric element 20 when the piezoelectric element 20 is driven and the vibration structure is deformed.

[0025] As described in the Background section, in piezoelectric elements, the piezoelectric body is generally formed on a diaphragm, and a laminated film is constructed so that the neutral plane of distortion of the piezoelectric element is located at the boundary between the piezoelectric body and the diaphragm or near the lower electrode on the diaphragm side of the piezoelectric body. Since the piezoelectric body is covered on the side opposite the diaphragm with a protective layer made of silicon nitride film, which has a large Young's modulus, if the laminated film is constructed so that the neutral plane of distortion is located on the lower electrode side, it is necessary to increase the rigidity of the diaphragm, and the rigidity of the vibration structure itself increases. As the rigidity increases, the vibration structure becomes thicker and harder to stack, making it difficult to achieve ultra-wideband.

[0026] Therefore, in the present invention, the laminated film 33 is configured so that the neutral plane of distortion in the vibration structure is located on the side of the protective layer 25 made of a silicon nitride film having a large Young's modulus, that is, on the opposite side to the gap 30 from the middle position B in the Z-axis direction of the cross section of the laminated film 33 shown in Fig. 7(b), that is, above in Fig. 7(b). Preferably, by positioning the neutral plane of distortion in the vicinity of the upper electrode 23, the displacement efficiency is improved, thereby improving the transmission and reception characteristics. The above-mentioned configuration allows the vibration structure to be made thin, soft, and light by ensuring rigidity on the protective layer 25 side, making it possible to realize an ultra-wideband. Therefore, it is possible to provide an ultrasonic device structure with high reliability by improving both the ultra-wideband and the transmission / reception efficiency. In terms of the configuration, the neutral plane of distortion in the vibration structure is located on the side of the protective layer 25 made of a silicon nitride film with a large Young's modulus, so that the vibration plate 16 and the lower electrode 21, which essentially function as a vibration plate, are made of a material having a smaller Young's modulus than the silicon nitride that constitutes the protective layer 25.

[0027] The diaphragm may be made of, for example, silicon, silicon dioxide, silicon nitride, or a combination thereof. The surfaces of the diaphragm 16 and the lower electrode 21 facing the piezoelectric body 22 are covered with an insulating layer 24, which is an insulating protective film. The insulating layer 24 may be 、 Silicon dioxide can be used, which is preferable because of its excellent insulating properties and adhesion to the lower electrode 21. As a modification of the above embodiment, it is possible to have only the lower electrode 21 function as the diaphragm without providing a layer that becomes the diaphragm 16. In this case, it is preferable to protect the lower electrode 21 on the side of the gap 30 with an insulating protective film. Silicon dioxide can be used as the protective film.

[0028] Next, an example of a method for manufacturing the PMUT chip 2 will be described. 8(a), the surface of a general silicon substrate 11 is thermally oxidized to form a SiO2 insulating film that will become the diaphragm 16 on the silicon substrate 11. At this time, the SiO2 insulating film is formed to have a thickness of 0.3 μm. Next, the lower electrode 21 is formed as shown in FIG. 8(b). At this time, a titanium oxide (TiO2) layer may be formed as an adhesive layer between the formed lower electrode 21 and the underlying diaphragm 16. A titanium film is formed on the SiO2 insulating film by sputtering, and then, using an RTA (Rapid Thermal Annealing) device, the titanium film is thermally oxidized at 650 to 800°C for 1 to 10 minutes in an O2 atmosphere to modify the titanium film into a titanium oxide film. This forms a titanium oxide adhesive layer made of a titanium oxide film on the diaphragm 16. At this time, the thickness of the formed titanium film is preferably 10 to 50 nm. The titanium oxide film may be formed by reactive sputtering, but it is preferable to use a thermal oxidation method at high temperature of the titanium film. The lower electrode 21 is formed by depositing, for example, platinum (Pt) to a thickness of 50 to 300 nm by sputtering. This process is a first electrode formation step.

[0029] A lead zirconate titanate (PZT) film is formed on the upper part of the lower electrode 21 by the so-called CSD (Chemical Solution Deposition) method, in which a precursor liquid made from lead acetate, a zirconium alkoxide compound, and a titanium alkoxide compound as starting materials is spin-coated, followed by drying, thermal decomposition, and crystallization. Next, in order to create a piezoelectric body 22 on the lower electrode 21, a solution was prepared with a composition ratio of Pb:Zr:Ti=110:52:48. To synthesize a specific precursor coating solution, lead acetate trihydrate, titanium isopropoxide, and zirconium isopropoxide were used as starting materials. The crystal water of lead acetate was dissolved in methoxyethanol and then dehydrated. The amount of lead was in excess of the stoichiometric composition. This was to prevent a decrease in crystallinity due to so-called lead loss during heat treatment.

[0030] Titanium isopropoxide and zirconium isopropoxide were dissolved in methoxyethanol, and the alcohol reaction and esterification reaction were carried out. The PZT precursor solution was synthesized by mixing with the methoxyethanol solution in which the lead acetate was dissolved. The PZT concentration of the synthesized PZT precursor solution was 0.5 mol / L. Using this solution, a PZT precursor film was formed on the lower electrode 21 by spin coating, and after the film formation, it was dried at 120°C and subjected to thermal decomposition treatment at 400°C.

[0031] After the thermal decomposition process of the third layer, the PZT precursor is subjected to a crystallization heat treatment (at 700°C) by RTA (rapid thermal processing). Since there is a concern that cracks may easily occur if the film thickness formed by one application is increased, in this embodiment, the film thickness after crystallization is adjusted to be within about 100 nm. After the above-mentioned crystallization is completed, the same process is repeated until a piezoelectric body 22 with a desired thickness is obtained. This process is a piezoelectric body formation step for forming the piezoelectric body 22. In this embodiment, the above-mentioned coating process is repeated about 5 to 40 times to form a piezoelectric body 22 with a thickness of about 0.5 to 4 μm, as shown in FIG. 8(c).

[0032] Next, as shown in Fig. 8(d), the upper electrode 23 is formed on the piezoelectric body 22. Specifically, first, a conductive oxide film, a SuRuO3 film, is formed on the piezoelectric body 22 by sputtering to a thickness of 40 nm. The substrate temperature during sputtering is set to 300°C. After that, a post-annealing process is performed in an oxygen atmosphere by RTA at 550°C / 300 sec, and then a metal film, a platinum film, is formed by sputtering to a thickness of 60 nm to form the upper electrode 23.

[0033] To obtain the piezoelectric element 20 processed into a predetermined shape, for example, after forming the upper electrode 23, a mask layer for use in etching is formed on the upper electrode 23 by patterning a photosensitive resist, and then dry etching or wet etching is performed. The photosensitive resist can be patterned by a well-known photolithography technique. That is, a photosensitive resist is applied onto the upper electrode 23 by a spin coater or roll coater, and then exposed to ultraviolet light using a glass photomask on which a desired pattern has been formed in advance. After that, a photosensitive resist mask layer is formed through the steps of pattern development, washing with water, and drying. Since the end inclination of the pattern of the formed photosensitive resist mask layer affects the inclined cross section during etching, the resist selection ratio (the ratio of the etching rate of the material to be etched to that of the mask material) may be selected according to the desired inclination angle. After the etching process, the photosensitive resist remaining on the film can be removed with a dedicated stripping solution or by oxygen plasma ashing.

[0034] Dry etching using reactive gas is often selected for the etching because of the stability of the shape. Examples of reactive gases used as etching gases include halogen-based gases such as chlorine and fluorine gases, and gases in which halogen-based gases are mixed with argon or oxygen. By changing the etching gas or etching conditions, it is possible to etch the upper electrode 23 and the piezoelectric body 22 continuously, or to perform etching in several steps by forming a new resist pattern once. Alternatively, instead of the above-mentioned method, the piezoelectric body 22 may be processed (etched) into a predetermined shape before the formation of the upper electrode 23, and then the upper electrode 23 may be formed only on the predetermined-shape piezoelectric body 22. The upper electrode 23 is patterned into a predetermined pattern by photolithography and etching.

[0035] Next, an insulating layer 24 is formed as an insulating film on the upper electrode 23. In this embodiment, the insulating layer 24 is a silicon oxide film formed by depositing a film of 0.4 to 1.0 μm using monosilane (SiH4) and nitrous oxide (N2O) gases as raw materials by plasma CVD. In the insulating film 24 thus formed, a contact hole 41 is opened by photolithography and etching as shown in FIG. 8(e), and a wiring material of Al-Cu (1 μm) / Ti (50 nm) is deposited by sputtering, and patterned by photolithography and etching to form a signal line 29 as shown in FIG. 8(f). 8(g), a silicon nitride film (Si3N4) is formed as a protective layer 25, and only the electrode terminal portion to be connected to the wiring 5 is opened. In this embodiment, the protective layer 25 is formed by forming a silicon nitride film (Si3N4) of about 0.5 to 1.5 μm using monosilane (SiH4) and nitrous oxide ammonia (NH3) gas as raw materials by a plasma CVD method. The opening of the electrode terminal portion is performed by photolithography and etching.

[0036] 8(h), the voids 30 are formed on the side of the silicon substrate 11 opposite to the piezoelectric elements 22, i.e., on the back surface side of the silicon substrate 11. Specifically, after the thickness of the silicon substrate 11 is adjusted to about 10 to 200 μm by back grinding and polishing, the surface of the silicon substrate 11 on which the piezoelectric elements 20 are formed (front surface, upper surface in the Z-axis direction) is attached to a support substrate. Next, a resist mask of a desired pattern is formed by photolithography on the back side (the surface on the lower side in the Z-axis direction) of the silicon substrate 11, and etching is performed on the silicon substrate 11. The etching process is easily performed using a silicon deep etcher, using the so-called Bosch process (a process in which etching with SF6 plasma and formation of a sidewall protective film with C4F8 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 by etching until the vibration membrane 16 is exposed, forming the void portion 30. This process is a cutting step for forming the void portion 30 on the side of the silicon substrate 11 opposite to the piezoelectric body 22. Finally, the wafer production process for the PMUT chip 2 is completed by peeling off the support wafer, dicing, etc.

[0037] As can be seen from the above, the piezoelectric element 20 can be formed in a simple process and has performance equivalent to that of bulk ceramics. In addition, a PMUT can be manufactured by subsequently etching away the back surface to form the void 30. According to the present invention, the laminated film 33 is configured so that the neutral plane of distortion of the laminated film 33 is located on the opposite side to the gap 30 from the midpoint B of the cross section of the laminated film at the center of the piezoelectric body 22, so that the vibration structure can be configured thin, soft, and light while ensuring rigidity on the protective layer 25 side. This makes it possible to provide a highly reliable ultrasonic device structure that has both an ultra-wideband and improved transmission and reception efficiency.

[0038] 7, the PMUT chip 2 of the present invention is configured so that the size of the cavity 30 in the X-axis direction is Cw, and the size of the piezoelectric body 22 in the X-axis direction is Pw, which is smaller than Cw. With this configuration, i.e., by arranging the piezoelectric body 22 inside the cavity 30, the rigidity of the vibration structure can be reduced, and an electromechanical conversion element and ultrasonic transducer can be provided that improves both the ultra-wideband and the transmission / reception efficiency and has high reliability. In this embodiment, as shown in FIG. 7, the PMUT chip 2 without the diaphragm 16 constitutes an electromechanical transducer element 18.

[0039] In the PMUT chip 2 of the present invention, the lower electrode 21 is made of a material having a Young's modulus smaller than that of the silicon nitride constituting the protective layer 25, such as silicon, silicon dioxide, silicon nitride, and the like, and combination With this configuration, the laminated film 33 can be configured so that the neutral plane of distortion in the vibration structure is located on the side of the protective layer 25 made of a silicon nitride film having a large Young's modulus, i.e., on the side opposite to the gap 30 from the midpoint B of the cross section of the laminated film 33.

[0040] Although a preferred embodiment of the present invention has been described above, the present invention is not limited to such a specific embodiment, and various modifications and variations are possible within the scope of the spirit of the present invention described in the claims, unless otherwise specifically limited in the above description. The effects described in the embodiments of the present invention are merely examples of the most preferable effects resulting from the present invention, and the effects of the present invention are not limited to those described in the embodiments of the present invention. [Explanation of symbols]

[0041] 1 Ultrasonic probe 2 Ultrasonic transducer (PMUT chip) 3 Support part 8 Acoustic Lens 9. Measurement Object 10. Ultrasound diagnostic equipment 11 Substrate (silicon substrate) 16 Diaphragm 18 Electromechanical transducer 21 1st electrode (lower electrode) 22 Piezoelectric 23 2nd electrode (upper electrode) 24 Insulating layer 25 Protective layer 30 Cavity 33 Laminated Film 61 Display section [Prior art documents] [Patent documents]

[0042] [Patent Document 1] JP 2017-118611 A [Patent Document 2] JP 2019-71361 A

Claims

1. A substrate, a first electrode, a piezoelectric body, and a second electrode, which are laminated in this order in a lamination direction; an insulating layer covering at least a portion of each of the first electrode, the piezoelectric body, and the second electrode; a protective layer covering the insulating layer; a gap portion formed in the substrate on a side opposite to the piezoelectric body with the first electrode interposed therebetween, An electromechanical transducer comprising a laminated film having the first electrode, the piezoelectric body, the second electrode, the insulating layer, and the protective layer, wherein the neutral plane of distortion of the laminated film is located on the opposite side to the gap portion from the midpoint in the stacking direction of a cross section of the laminated film at the center of a direction perpendicular to the stacking direction of the piezoelectric body.

2. 2. The electromechanical transducer according to claim 1, An electromechanical transducer element, characterized in that the size of the piezoelectric body in a direction perpendicular to the stacking direction is smaller than the size of the gap in the direction perpendicular to the stacking direction.

3. 3. The electromechanical transducer according to claim 1, The electromechanical transducer according to claim 1, wherein the first electrode is made of a material having a smaller Young's modulus than silicon nitride.

4. 4. The electromechanical transducer according to claim 1, An electromechanical transducer, wherein a plurality of the piezoelectric bodies and a plurality of the voids are arranged in an array.

5. 5. An ultrasonic transducer comprising the electromechanical conversion element according to claim 1 and a diaphragm.

6. 6. A method for manufacturing a ultrasonic transducer comprising: a first ultrasonic transducer and an acoustic lens; and a support portion for supporting the ultrasonic transducer and the acoustic lens, An ultrasonic probe characterized in that a voltage is applied to the electromechanical conversion element to vibrate the diaphragm, thereby emitting ultrasonic waves toward a measurement object, and detecting the vibration of the ultrasonic waves reflected by the measurement object.

7. 7. An ultrasonic diagnostic apparatus comprising the ultrasonic probe according to claim 6 and a display unit for displaying the shape of the object to be measured.

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