MEMS ultrasonic transducer and ultrasonic distance sensor

Abstract

A MEMS ultrasonic transducer according to the present disclosure comprises: a diaphragm including a material in which the frequency at which the amplitude of nonlinear vibration peaks shifts to a value higher than the resonance frequency of linear vibration; a first insulation part laminated on the diaphragm; a lower electrode laminated on a first insulation thin film; a piezoelectric thin film that is laminated on the lower electrode, contracts due to a voltage, and causes the diaphragm to vibrate; an upper electrode laminated on the piezoelectric thin film; a second insulation part laminated on the upper electrode; and an initial strain that is formed on the diaphragm and shifts the frequency at which the amplitude of the nonlinear vibration peaks to a value lower than the resonance frequency of the diaphragm.

Description

MEMS ultrasonic transducer and ultrasonic distance sensor

[0001] The present disclosure relates to a MEMS ultrasonic transducer and an ultrasonic distance sensor that can be used as a distance sensor for monitoring the periphery of a vehicle or the like, a gesture sensor, or the like.

[0002] Ultrasonic transducers are used as distance sensors for applications such as perimeter monitoring in automobiles and non-contact operation of devices. A commonly used ultrasonic transducer is a bulk lead zirconate titanate (PZT: Pb(Zr,Ti)O ３ ) is used as an actuator. However, ultrasonic transducers using bulk lead zirconate titanate (PZT) as an actuator have limitations on miniaturization due to the machining accuracy. For this reason, MEMS (Micro Electro Mechanical Systems) ultrasonic transducers have been developed as ultrasonic transducers that can be miniaturized.

[0003] A typical ultrasonic transducer has a diaphragm structure using a thin-film diaphragm (membrane). This diaphragm structure uses a material that exhibits a hard spring effect in its frequency characteristics, such as a silicon diaphragm. Here, the hard spring effect refers to a characteristic in which, at low amplitudes, the peak frequency and the resonant frequency are approximately the same, but as the amplitude increases, nonlinear frequency characteristics appear, the peak frequency shifts to the higher frequency side, and the amplitude changes rapidly near the peak frequency. Patent Document 1 discloses an ultrasonic transducer with a diaphragm structure. The ultrasonic transducer has a multilayer diaphragm on which a piezoelectric thin film, an electrode, and an insulating thin film are layered. The interaction between the piezoelectric thin film and the diaphragm enables ultrasonic measurement and output.

[0004] Japanese Patent Application Laid-Open No. 2006-319945

[0005] Similarly, in a MEMS ultrasonic transducer, when a voltage is applied, the piezoelectric thin film contracts to generate ultrasonic waves from the diaphragm diaphragm. Conventional ultrasonic transducers can generate sufficiently large vibrations without driving the diaphragm (resonant driving) at a frequency where the vibrations peak. However, to generate large vibrations with a MEMS ultrasonic transducer, the diaphragm must be resonantly driven to vibrate with a nonlinear frequency characteristic. For example, when using a material with a hard spring effect in its frequency characteristic, such as silicon, in a frequency band where the vibration amplitude of the diaphragm is particularly large, a hysteresis phenomenon occurs in which the frequency at which the amplitude is maximum shifts between an up-sweep in which the driving frequency is swept toward a higher frequency and a down-sweep in which the driving frequency is swept toward a lower frequency. As a result, MEMS ultrasonic transducers have the problem of being unable to generate stable vibrations at large amplitudes in nonlinear vibrations.

[0006] The present disclosure has been made to solve such problems, and has an object to provide a MEMS ultrasonic transducer and an ultrasonic distance sensor that are capable of stable vibration with large amplitude.

[0007] The MEMS ultrasonic transducer according to the present disclosure comprises a diaphragm containing a material that shifts the frequency at which the amplitude of the nonlinear vibration peaks to a value higher than the resonant frequency, a first insulating portion laminated on the diaphragm, a lower electrode laminated on the first insulating thin film, a piezoelectric thin film laminated on the lower electrode that contracts in response to voltage and vibrates the diaphragm, an upper electrode laminated on the piezoelectric thin film, a second insulating portion laminated on the upper electrode, and an initial strain formed on the diaphragm that shifts the frequency at which the amplitude of the nonlinear vibration peaks to a value lower than the resonant frequency of the diaphragm.

[0008] According to the MEMS ultrasonic transducer and ultrasonic distance sensor according to the present disclosure, by forming an initial strain, it is possible to generate large-amplitude, stable vibrations in a frequency band that causes nonlinear vibrations.

[0009] FIG. 1 is a perspective view illustrating a configuration of a MEMS ultrasonic transducer according to a first embodiment of the present disclosure. FIG. 2 is a cross-sectional view illustrating a configuration of a MEMS ultrasonic transducer according to the first embodiment of the present disclosure. FIG. 3 is a graph illustrating amplitude and frequency characteristics when there is no initial strain in the MEMS ultrasonic transducer according to the first embodiment of the present disclosure. FIG. 4 is a perspective view illustrating a shape of initial strain in the MEMS ultrasonic transducer according to the first embodiment of the present disclosure. FIG. 5 is a graph illustrating amplitude and frequency characteristics of the MEMS ultrasonic transducer according to the first embodiment of the present disclosure. FIG. 6 is a graph illustrating a correlation between static force and displacement of the MEMS ultrasonic transducer according to the first embodiment of the present disclosure. FIG. 7 is a perspective view illustrating a configuration of a MEMS ultrasonic transducer according to a second embodiment of the present disclosure. FIG. 8 is a cross-sectional view illustrating a configuration of a MEMS ultrasonic transducer according to a third embodiment of the present disclosure. FIG. 9 is a schematic configuration diagram illustrating a measurement method of an ultrasonic distance sensor according to a fourth embodiment of the present disclosure. FIG. 10 is a graph illustrating transmission frequency characteristics of the ultrasonic distance sensor according to the fourth embodiment of the present disclosure. FIG. 11 is a graph illustrating a correlation between DC bias and resonant frequency during reception of the ultrasonic distance sensor according to the fourth embodiment of the present disclosure.

[0010] Hereinafter, a MEMS ultrasonic transducer and an ultrasonic distance sensor according to the present disclosure will be described with reference to the drawings.

[0011] First Embodiment. FIG. 1 is a perspective view illustrating a configuration of a MEMS ultrasonic transducer according to a first embodiment of the present disclosure. As illustrated in FIG. 1 , the MEMS ultrasonic transducer 100 according to the first embodiment of the present disclosure includes a diaphragm 119 including a material that shifts the frequency at which the amplitude of nonlinear vibration peaks to a value higher than the resonant frequency of linear vibration, a first insulating portion stacked on the diaphragm 119, a lower electrode 114 stacked on the first insulating portion, a piezoelectric thin film 115 stacked on the lower electrode 114 and contracting in response to a voltage to vibrate the diaphragm 119, an upper electrode 116 stacked on the piezoelectric thin film 115, and a second insulating portion stacked on the upper electrode 116. Here, the first insulating portion is the insulating thin film 110. The second insulating portion is an upper first insulating thin film 117 and an upper second insulating thin film 118, which are omitted in FIG. 1 . The piezoelectric thin film 115 has multiple legs 202 whose ends extend toward the outer edge of the first insulating portion.

[0012] FIG. 2 is a cross-sectional view showing the configuration of a MEMS ultrasonic transducer according to the first embodiment of the present disclosure. FIG. 2 is also a cross-section taken along the line A-A′ in FIG. 1 . Below, an example in which silicon is used as the diaphragm material is shown. As shown in FIG. 2 , the MEMS ultrasonic transducer 100 according to the first embodiment of the present disclosure has a diaphragm structure in which the SOI (Silicon on Insulator) substrate support layer 113 of the diaphragm 119 is processed from the backside using deep reactive ion etching (DRIE) or the like, with the SOI intermediate oxide film 112 serving as an etching stop layer, to form a void 101. The processing process for the diaphragm 119 is performed after a film structure is formed on the diaphragm 119. Here, the size of the diaphragm 119 may be, for example, approximately 800 μm in diameter in a plane perpendicular to the stacking direction. The SOI active layer 111 above the void 101 becomes the diaphragm 103.

[0013] As shown in FIG. 2 , an insulating thin film 110 is laminated on the SOI active layer 111. While various methods for forming the insulating thin film 110 are available, thermal oxidation, which can minimize surface roughness, is preferred. A lower electrode 114, a piezoelectric thin film 115, and an upper electrode 116 are laminated on the insulating thin film 110. Here, the lower electrode 114 and the upper electrode 116 are preferably made of a laminated film of titanium (Ti) and platinum (Pt), which are commonly used for piezoelectric thin films. However, other laminated films may be used as long as they have sufficient conductivity as an electrode and can ensure good adhesion to the substrate. Furthermore, an oxide electrode film, such as a strontium oxide (SrO) film, which is believed to be effective in reducing polarization fatigue, may be interposed between the upper electrode 116 and the piezoelectric thin film 115.

[0014] The piezoelectric thin film 115 is made of a material such as lead zirconate titanate (PZT: Pb(Zr,Ti)O), aluminum nitride (AlN), or potassium sodium niobate (KNN: (K,Na)NbO). An upper first insulating thin film 117 and an upper second insulating thin film 118, which form a second insulating portion, are laminated on the upper electrode 116. The upper first insulating thin film 117 provides electrical insulation between the lower electrode 114 and the upper electrode 116, while the upper second insulating thin film 118 serves as a protective film for the entire structure, including the upper electrode 116. The upper first insulating thin film 117 and the upper second insulating thin film 118 are silicon oxide or silicon nitride films formed by plasma CVD or sputtering, and are formed to have appropriate film stress, as described below. While the second insulating portion is shown here as consisting of two layers, the upper first insulating thin film 117 and the upper second insulating thin film 118, it may be a single layer or two or more layers.

[0015] In the MEMS ultrasonic transducer 100 according to the first embodiment of the present disclosure, when a voltage is applied between the lower electrode 114 and the upper electrode 116, the piezoelectric thin film 115 contracts, and the contraction of the piezoelectric thin film 115 causes the diaphragm 103 to perform a bending motion. When a voltage is applied at a frequency close to the resonant frequency of the diaphragm 103, the diaphragm 103 resonates and vibrates. This allows ultrasonic waves to be generated. The ultrasonic transducer 100 can also be used as an ultrasonic sensor, and can measure ultrasonic waves by acquiring the vibration of the diaphragm 103, which is vibrated by ultrasonic waves, from the charge generated in the piezoelectric thin film 115.

[0016] The piezoelectric thin film 115 is formed of a vibrating portion 201 located in the center of the diaphragm 119 and legs 202 having a plurality of ends extending from the vibrating portion 201 toward the outer edge. An upper electrode 116 is formed on a part of the leg 201, and by applying a voltage to the upper electrode 116 on the vibrating portion 201, the vibrating portion 201 is driven to vibrate the diaphragm 119.

[0017] Here, the specific dimensions of the MEMS ultrasonic transducer 100 are, for example, a thickness of the vibration plate 103 of several μm, a thickness of the insulating thin film 110 of 0.1 to 1 μm, a thickness of the lower electrode 114 and the upper electrode 116 of about 0.1 μm, a thickness of the piezoelectric thin film 115 of about several μm, and a thickness of the upper first insulating thin film 117 and the upper second insulating thin film 118 of about 0.1 to 1 μm.

[0018] FIG. 3 is a graph showing the amplitude and frequency characteristics of the MEMS ultrasonic transducer according to the first embodiment of the present disclosure when there is no initial strain. This graph shows the frequency characteristics of a material with a hard spring effect, which shifts the frequency at which the amplitude of nonlinear vibration peaks to a value higher than the resonant frequency. As shown in FIG. 3 , in the linear range at low amplitudes, the peak frequency coincides with the resonant frequency f0 of the diaphragm 119. However, as the amplitude increases and enters the nonlinear range, the peak frequency shifts toward higher frequencies, and the amplitude changes rapidly near the peak frequency. Further increasing the amplitude of the diaphragm 119 results in a hysteresis phenomenon, in which the frequencies at which the amplitude peaks shift between up-swings and down-swings. In FIG. 3 , in the frequency band where hysteresis occurs, when the transducer is driven at the same frequency without up-swings, the displacement exhibits the frequency characteristics of the down-swing, shown by the dotted line, and it is not possible to drive at a large amplitude. Therefore, the maximum usable amplitude is lower than the peak frequency of the down-swing.

[0019] Near the peak frequency of the down sweep, even a slight fluctuation causes a drastic change in amplitude, so the maximum usable amplitude is at a drive frequency shifted lower than the peak frequency of the down sweep. Thus, for materials with frequency characteristics of a hard spring effect, the peak amplitude and its frequency differ significantly between the up sweep and the down sweep, making stable vibration at large amplitudes difficult. Here, a large amplitude in this disclosure refers to an amplitude that results in nonlinear vibration, and is greater than the amplitude at the resonant frequency of linear vibration.

[0020] FIG. 4 is a perspective view showing the shape of the initial strain of the MEMS ultrasonic transducer according to the first embodiment of the present disclosure. The initial strain 200 shifts the frequency at which the amplitude of the nonlinear vibration peaks to a value lower than the resonant frequency of the diaphragm 119. This initial strain 200 is formed by adjusting the film stress and film thickness of the first insulating portion, the piezoelectric thin film 115, and the second insulating portion. In this disclosure, the MEMS ultrasonic transducer 100 has a size of approximately 800 μm in diameter, while the size of the initial strain 200 is approximately several μm in the stacking direction. Therefore, the initial strain 200 shown in FIG. 4 is a deformation amount that is approximately 100 times larger. In this case, if the film stress of the first insulating portion, the piezoelectric thin film 115, and the second insulating portion is adjusted to be compressive stress, a convex-shaped initial strain 200 as shown in FIG. 4 can be formed. Alternatively, if the film stress of the first insulating portion, the piezoelectric thin film 115, and the second insulating portion is adjusted to be tensile stress, a concave-shaped initial strain 200 can be formed. The shape of the initial strain 200 of the MEMS ultrasonic transducer 100 according to the first embodiment of the present disclosure may be any shape as long as the cross section is line-symmetrical with respect to the central axis along the stacking direction.

[0021] FIG. 5 is a graph showing the amplitude and frequency characteristics of the MEMS ultrasonic transducer according to the first embodiment of the present disclosure. The horizontal axis represents the drive frequency, and the vertical axis represents the diaphragm amplitude. To drive the diaphragm 119, a voltage is applied between the lower electrode 114 and the upper electrode 116. In FIG. 5( a), characteristic A, which shows the lowest amplitude in the graph, has a peak frequency that coincides with the resonant frequency f0 of the diaphragm 119. When the drive voltage is increased to increase the amplitude and cause nonlinear vibration, the vibration of the diaphragm 119 shifts to characteristic B and characteristic C. In characteristic B and characteristic C, the frequency at which the amplitude peaks shifts from the resonant frequency to a lower frequency.

[0022] As shown in Figure 5(a), when the amplitude of the diaphragm 119 is further increased, the peak frequency in characteristic D shifts further to the lower frequency side, and hysteresis occurs between the up-sweep and the down-sweep. When hysteresis occurs in the vibration of the diaphragm 119, the frequencies at which the amplitude peaks differ between the up-sweep and the down-sweep. In Figure 5(a), when the amplitude of the diaphragm 119 is further increased, characteristic E is obtained, and the frequency that had been shifted to the lower frequency side shifts to the higher frequency side. This is a frequency characteristic that combines a soft spring effect that shifts the frequency at which the amplitude of the nonlinear vibration peaks to a value lower than the resonant frequency of the diaphragm 119 and a hard spring effect that shifts the frequency at which the amplitude of the nonlinear vibration peaks to a value higher than the resonant frequency of the diaphragm 119. In other words, when the frequency at which the amplitude of the nonlinear vibration peaks shifts to a value lower than the resonant frequency of the diaphragm, the initial strain 200 increases the frequency at which the amplitude of the nonlinear vibration peaks due to the increase in the amplitude of the nonlinear vibration, thereby generating a frequency characteristic that combines a soft spring effect and a hard spring effect.

[0023] As shown in FIG. 5B, the frequency characteristics in which the soft spring effect and hard spring effect are combined exhibit hysteresis on both the low-frequency and high-frequency sides, but the amplitude at which the up-sweep and down-sweep overlap is larger than that shown in FIG. 3. Although FIG. 5B and FIG. 3 have the same resonant frequency and the same vertical scale, the maximum usable displacement in FIG. 5B is approximately twice that of FIG. 3, and the usable bandwidth is also wider. Therefore, the MEMS ultrasonic transducer 100 according to the first embodiment of the present disclosure is formed by the film stress and film thickness of the first insulating section, the piezoelectric thin film, and the second insulating section, and has an initial strain 200 that shifts the frequency at which the amplitude of the nonlinear vibration peaks to a value lower than the resonant frequency of the diaphragm. This results in a combined characteristic of the soft spring effect and the hard spring effect, enabling stable vibration with large amplitude. Furthermore, because large amplitudes are possible over a wide bandwidth, ultrasonic waves with high sound pressure can be transmitted over a wide bandwidth. Note that, as defined in this disclosure, ultrasonic waves are sound waves with frequencies of approximately 20 kHz to 100 kHz.

[0024] FIG. 6 is a graph showing the correlation between static force and displacement of the MEMS ultrasonic transducer according to the first embodiment of the present disclosure. The horizontal axis of FIG. 6 represents the displacement ΔZ of the diaphragm 119 in the Z direction from the initial position, where the Z direction is parallel to the stacking direction. The vertical axis represents the static force F, which is the force acting on the diaphragm 119. In resonant driving that is not a nonlinear vibration, the vibration characteristics of the diaphragm 119 are linear vibrations that follow Hooke's law. Here, nonlinear vibration in the present disclosure refers to vibrations that have a term including a nonlinear coefficient in the vibration characteristic equation and have an amplitude greater than the amplitude of the resonant frequency.

[0025] As shown in FIG. 6 , the correlation between F and ΔZ is asymmetric on both the horizontal and vertical axes. Here, the deformation of the initial strain 200 shown in FIG. 4 where ΔZ > 0 is the deformation of the upper side of the diaphragm 119, which can be approximated by the frequency characteristics of the hard spring effect. The deformation where ΔZ < 0 is the deformation of the lower side of the diaphragm 119, and the slope of the static force F is very small near ΔZ = 0. Here, a small slope of the static force F means that a small force can cause a large deformation. In other words, by forming the initial strain 200, the lower side of the diaphragm 119 has a structure that is easily deformed. Note that as the amount of deformation increases, the slope of the graph also increases for the deformation of the lower side of the diaphragm 119.

[0026] When the diaphragm is driven with the force shown in FIG. 7A, the lower displacement is greater than the upper displacement, resulting in a soft spring effect in the frequency characteristics. When the diaphragm is driven with the force shown in FIG. 7B, the soft spring effect is evident at relatively low displacements in the nonlinear vibration. However, as the amplitude increases, a hard spring effect appears, in which the frequency at which the amplitude peaks increases as the displacement increases. Therefore, the frequency characteristics of the MEMS ultrasonic transducer 100 having the initial strain 200 are a composite characteristic of the soft spring effect and the hard spring effect. However, for the characteristics that can vibrate with the large displacement shown in FIG. 5B, there is an appropriate range of initial strain for the resonant frequency. When the initial strain 200 is small, the soft spring effect does not appear, resulting in a frequency characteristic that has only the hard spring effect. When the initial strain is excessive, the diaphragm 119 buckles, preventing normal vibration. Therefore, it is important that the initial strain 200 is large enough to obtain the frequency characteristics shown in FIG. 5, in which the soft spring effect and the hard spring effect are combined.

[0027] As described above, the MEMS ultrasonic transducer 100 according to the first embodiment of the present disclosure comprises a diaphragm 119 containing a material that shifts the frequency at which the amplitude of the nonlinear vibration peaks to a value higher than the resonant frequency of the linear vibration, a first insulating portion laminated on the diaphragm 119, a lower electrode 114 laminated on the first insulating thin film, a piezoelectric thin film 115 laminated on the lower electrode 114 and contracting in response to a voltage, an upper electrode 116 laminated on the piezoelectric thin film 115, a second insulating portion laminated on the upper electrode 116, and an initial strain 200 formed on the diaphragm that shifts the frequency at which the amplitude of the nonlinear vibration peaks to a value lower than the resonant frequency of the diaphragm.

[0028] The MEMS ultrasonic transducer 100 according to the first embodiment of the present disclosure has the initial strain 200, and therefore can vibrate with a frequency characteristic that combines a soft spring effect that shifts the frequency at which the amplitude of the nonlinear vibration peaks to a value lower than the resonant frequency of the diaphragm 119, and a hard spring effect that shifts the frequency at which the amplitude of the nonlinear vibration peaks to a value higher than the resonant frequency of the diaphragm 119. Therefore, the MEMS ultrasonic transducer 100 according to the first embodiment of the present disclosure is capable of stable vibration with a large amplitude.

[0029] In the MEMS ultrasonic transducer 100 according to the first embodiment of the present disclosure, an example in which silicon is used as the material for the diaphragm 119 has been described. However, the material for the diaphragm 119 may be a material that has a hard spring effect that shifts the frequency at which the amplitude of the nonlinear vibration peaks to a value higher than the resonant frequency.

[0030] Second Embodiment In a second embodiment, the same components as those in the first embodiment of the present disclosure are designated by the same reference numerals, and descriptions of the same or corresponding parts will be omitted. Hereinafter, a MEMS ultrasonic transducer 100b according to the second embodiment will be described with reference to the drawings.

[0031] 7 is a perspective view showing a configuration of a MEMS ultrasonic transducer according to a second embodiment of the present disclosure. Similar to the first embodiment, the MEMS ultrasonic transducer 100b according to the second embodiment of the present disclosure includes a diaphragm 119 including a material that shifts the frequency at which the amplitude of the nonlinear vibration peaks to a value higher than the resonant frequency of the linear vibration, a first insulating portion laminated on the diaphragm 119, a lower electrode 114 laminated on the first insulating thin film, a piezoelectric thin film 115 that is laminated on the lower electrode 114 and contracts in response to a voltage, an upper electrode 116 laminated on the piezoelectric thin film 115, a second insulating portion laminated on the upper electrode 116, and an initial strain 200 that is formed on the diaphragm and shifts the frequency at which the amplitude of the nonlinear vibration peaks to a value lower than the resonant frequency of the diaphragm.

[0032] In the MEMS ultrasonic transducer 100b according to the second embodiment of the present disclosure, the end portions of the piezoelectric thin film 115 are thicker than those shown in FIG. 1 of the first embodiment. Also, as shown in FIG. 7 , while the first embodiment has four legs, the MEMS ultrasonic transducer 100b has eight legs 202 in FIG. 7( a ) and twelve legs 202 in FIG. 7( b ). In this way, the rigidity of the MEMS ultrasonic transducer 100b can be adjusted by adjusting at least one of the number and width of the legs. Furthermore, by combining the membrane stress with the number and width of the leg ends, a MEMS ultrasonic transducer can be realized in which an initial strain 200 is formed, resulting in a frequency characteristic that combines an optimal soft spring effect and a hard spring effect.

[0033] As described above, the MEMS ultrasonic transducer 100b according to the second embodiment of the present disclosure has the initial strain 200 as in the first embodiment, and therefore can vibrate with a frequency characteristic that combines a soft spring effect that shifts the frequency at which the amplitude of the nonlinear vibration peaks to a value lower than the resonant frequency of the diaphragm 119, and a hard spring effect that shifts the frequency at which the amplitude of the nonlinear vibration peaks to a value higher than the resonant frequency of the diaphragm 119. Therefore, the MEMS ultrasonic transducer 100b according to the second embodiment of the present disclosure is capable of stable vibration with a large amplitude, as in the first embodiment.

[0034] Furthermore, the rigidity of the MEMS ultrasonic transducer 100b according to the second embodiment of the present disclosure can be adjusted by adjusting at least one of the number and width of the legs.

[0035] Furthermore, by combining the membrane stress with the number and width of the leg ends, it is possible to realize a MEMS ultrasonic transducer in which an initial strain 200 is formed, which results in frequency characteristics that combine an optimal soft spring effect and a hard spring effect.

[0036] Third Embodiment In a third embodiment, the same components as those in the first embodiment of the present disclosure are designated by the same reference numerals, and descriptions of the same or corresponding parts will be omitted. Hereinafter, a MEMS ultrasonic transducer 100c according to the third embodiment will be described with reference to the drawings.

[0037] Similar to the first embodiment, the MEMS ultrasonic transducer 100c according to the third embodiment of the present disclosure comprises a diaphragm 119 containing a material that shifts the frequency at which the amplitude of the nonlinear vibration peaks to a value higher than the resonant frequency of the linear vibration, a first insulating portion laminated on the diaphragm 119, a lower electrode 114 laminated on the first insulating thin film, a piezoelectric thin film 115 laminated on the lower electrode 114 and contracting in response to voltage, an upper electrode 116 laminated on the piezoelectric thin film 115, a second insulating portion laminated on the upper electrode 116, and an initial strain 200 formed on the diaphragm that shifts the frequency at which the amplitude of the nonlinear vibration peaks to a value lower than the resonant frequency of the diaphragm.

[0038] FIG. 8 is a cross-sectional view showing the configuration of a MEMS ultrasonic transducer according to a third embodiment of the present disclosure. As shown in FIG. 8 , the MEMS ultrasonic transducer 100 c according to the third embodiment of the present disclosure includes a strain adjustment film 301 that is provided in the gap 101 of the diaphragm 119 and adjusts the magnitude of the initial strain 200. The strain adjustment film 301 is formed to have compressive or tensile stress in order to adjust the initial strain 200 after processing the MEMS ultrasonic transducer. The film may be formed during a wafer process, or may be formed individually on each chip after chip separation to compensate for processing variations in the initial strain. Alternatively, the magnitude of the initial strain may be adjusted by the strain adjustment film 301 alone, without adjusting the initial strain using the film stress of the upper insulating films 117 and 118.

[0039] The upper first insulating thin film 117 and the upper second insulating thin film 118 require patterning after deposition for electrode openings and the like, but the strain adjustment film 301 does not require patterning. Therefore, providing the strain adjustment film 301 simplifies the film deposition process and increases the selectivity of film stress and film thickness. By forming the strain adjustment film 301 and adjusting the magnitude of the initial strain, it is possible to realize a MEMS ultrasonic transducer having frequency characteristics that combine a soft spring effect and a hard spring effect with little variation.

[0040] As described above, the MEMS ultrasonic transducer 100c according to the third embodiment of the present disclosure has the initial strain 200 as in the first embodiment, and therefore can vibrate with a frequency characteristic that combines a soft spring effect that shifts the frequency at which the amplitude of the nonlinear vibration peaks to a value lower than the resonant frequency of the diaphragm 119, and a hard spring effect that shifts the frequency at which the amplitude of the nonlinear vibration peaks to a value higher than the resonant frequency of the diaphragm 119. Therefore, the MEMS ultrasonic transducer 100c according to the third embodiment of the present disclosure is capable of stable vibration with a large amplitude, as in the first embodiment.

[0041] Furthermore, the MEMS ultrasonic transducer 100c according to the third embodiment of the present disclosure includes a strain adjustment film 301 that is provided in the gap 101 of the diaphragm 119 and that adjusts the magnitude of the initial strain 200. This simplifies the film formation process and increases the selectivity of the film stress and film thickness.

[0042] Furthermore, by forming the strain adjustment film 301 and adjusting the magnitude of the initial strain, it is possible to realize a MEMS ultrasonic transducer having frequency characteristics that combine a soft spring effect and a hard spring effect with little variation.

[0043] Fourth Embodiment In a fourth embodiment, an ultrasonic distance sensor 701 equipped with the MEMS ultrasonic transducer according to the first, second, and third embodiments will be described with reference to the drawings. Note that the same components as those in the first embodiment of the present disclosure are designated by the same reference numerals, and descriptions of the same or corresponding parts will be omitted. Below, an example equipped with a MEMS ultrasonic transducer 100 will be described.

[0044] 9 is a schematic diagram illustrating a measurement method of an ultrasonic distance sensor according to a fourth embodiment of the present disclosure. An ultrasonic distance sensor 701 measures distance using time of flight. In FIG. 9 , a solid line represents a transmitted wave 703, and a dashed line represents a reflected wave 704 from an object to be measured 702. As shown in FIG. 9 , an ultrasonic wave transmitted as a transmitted wave from the ultrasonic distance sensor 701 is reflected by the object to be measured 702 and received by the ultrasonic distance sensor 701 as a reflected wave 704.

[0045] The ultrasonic distance sensor 701 according to the fourth embodiment of the present disclosure includes a MEMS ultrasonic transducer and a measurement unit (not shown) that measures the distance to an object based on the time difference between transmission and reception of ultrasonic waves.

[0046] The ultrasonic waves that reach the ultrasonic distance sensor 701 vibrate the diaphragm 103 of the MEMS ultrasonic transducer 100. The vibration of the diaphragm 103 is received as an electrical signal by the piezoelectric thin film 115 arranged on the diaphragm. When the distance between the ultrasonic distance sensor 701 and the object to be measured 702 is L, the time taken from transmission to reception of the ultrasonic waves is t, and the speed of sound is c, L can be calculated using equation (1).

[0047]

[0048] At this time, the measured time t is the time required for the round trip between transmission and reception, so the distance L is given by equation (1).

[0049] 10 is a graph showing transmission frequency characteristics of an ultrasonic distance sensor according to a fourth embodiment of the present disclosure. The ultrasonic distance sensor 701 according to the fourth embodiment of the present disclosure uses a MEMS ultrasonic transducer 100 including: a diaphragm 119 including a material that shifts the frequency at which the amplitude of the nonlinear vibration peaks to a value higher than the resonant frequency; a first insulating portion laminated on the diaphragm 119; a lower electrode 114 laminated on the first insulating thin film; a piezoelectric thin film 115 that is laminated on the lower electrode 114 and contracts in response to a voltage to vibrate the diaphragm 119; an upper electrode 116 laminated on the piezoelectric thin film 115; and a second insulating portion laminated on the upper electrode 116. The MEMS ultrasonic transducer 100 is formed by the film stresses and film thicknesses of the first insulating portion, the piezoelectric thin film 115, and the second insulating portion, and has an initial strain 200 that shifts the frequency at which the amplitude of the nonlinear vibration peaks to a value lower than the resonant frequency of the diaphragm. For this reason, the frequency characteristics are a combination of the hard spring effect and soft spring effect of the MEMS ultrasonic transducer 100, and the ultrasonic distance sensor 701 can vibrate at a frequency fs as shown in Fig. 10. At this time, because of the combined characteristics of the hard spring effect and soft spring effect, the frequency fs of the transmitted ultrasonic waves is lower than the resonant frequency f0.

[0050] 11 is a graph showing the correlation between the DC bias and the resonant frequency during reception in the ultrasonic distance sensor according to the fourth embodiment of the present disclosure. This is the correlation between the DC bias and the resonant frequency of the linear vibration as shown in FIG. 5A , not that of the nonlinear vibration. This DC bias refers to a DC voltage applied to the lower electrode 114, and applying such a DC voltage can change the resonant frequency of the diaphragm 119. Therefore, by applying a DC voltage to the lower electrode of the MEMS ultrasonic transducer 100, the resonant frequency at which reflected ultrasonic waves are received in the ultrasonic distance sensor 701 can be changed.

[0051] In distance measurement using the ultrasonic distance sensor 701 according to the fourth embodiment of the present disclosure, a transmission wave 703 with a large amplitude and a frequency fs is transmitted. After transmitting the ultrasonic wave, a DC bias Vs shown in FIG. 11 is applied to the lower electrode 114 to tune the resonant frequency of the diaphragm 119 to fs. A reflected wave 704 with a frequency fs reflected by the object 702 vibrates the MEMS transducer at the changed resonant frequency and is received as an electrical signal by a piezoelectric thin film arranged on the diaphragm.

[0052] As described above, the ultrasonic distance sensor 701 according to the fourth embodiment of the present disclosure includes the MEMS ultrasonic transducer 100 and a measurement unit that measures the distance to an object based on the time difference between transmission and reception of ultrasonic waves, and the measurement unit transmits ultrasonic waves at a transmission frequency that is lower than the resonant frequency using the MEMS ultrasonic transducer 100, applies a DC voltage to the lower electrode to change the resonant frequency of the diaphragm 119 to the transmission frequency, and receives ultrasonic waves reflected from the object. Therefore, the ultrasonic distance sensor 701 according to the fourth embodiment of the present disclosure can improve the generated sound pressure and sensitivity by using a DC bias to match the resonant frequency of the diaphragm 119 after transmitting ultrasonic waves with the transmission frequency, thereby improving the detection distance and distance accuracy.

[0053] The configurations described in the above embodiments are merely examples of the contents of the present disclosure, and may be combined with other known technologies. Furthermore, parts of the configurations may be omitted or modified without departing from the scope of the present disclosure.

[0054] 100 100b 100c MEMS ultrasonic transducer, 101 gap, 102 support, 103 diaphragm, 110 insulating thin film, 111 SOI substrate active layer, 112 SOI substrate intermediate oxide film, 113 SOI substrate support layer, 114 lower electrode, 115 piezoelectric thin film, 116 upper electrode, 117 upper first insulating thin film, 118 upper second insulating thin film, 119 diaphragm, 200 initial strain, 201 vibrating part, 202 leg part, 301 strain adjustment film, 701 ultrasonic distance sensor, 702 measurement object, 703 transmitted wave, 704 received wave

Claims

1. A diaphragm containing a material in which the frequency at which the amplitude of nonlinear vibration peaks shifts to a value higher than the resonance frequency of linear vibration, A first insulating portion laminated on the diaphragm, A lower electrode stacked on the first insulating portion, A piezoelectric thin film is laminated on the lower electrode, which contracts when a voltage is applied and causes the diaphragm to vibrate, An upper electrode stacked on the piezoelectric thin film, A second insulating portion is laminated on the upper electrode, A MEMS ultrasonic transducer comprising an initial strain formed on the diaphragm, which shifts the frequency at which the amplitude of the nonlinear vibration peaks to a value lower than the resonant frequency of the diaphragm.

2. The MEMS ultrasonic transducer according to claim 1, characterized in that the initial strain is formed by adjusting the film stress and film thickness of the first insulating portion, the piezoelectric thin film, and the second insulating portion.

3. The MEMS ultrasonic transducer according to claim 1, characterized in that when the initial strain shifts to a value such that the frequency at which the amplitude of the nonlinear vibration peaks is lower than the resonant frequency of the diaphragm, the frequency at which the amplitude of the nonlinear vibration peaks is increased by increasing the amplitude of the nonlinear vibration.

4. The MEMS ultrasonic transducer according to claim 1, characterized in that the initial strain has a cross-sectional shape that is symmetrical with respect to the central axis along the stacking direction.

5. The MEMS ultrasonic transducer according to claim 2, characterized in that the initial strain has a cross-sectional shape that is symmetric with respect to the central axis along the stacking direction.

6. The MEMS ultrasonic transducer according to claim 3, characterized in that the initial strain has a cross-sectional shape that is symmetrical with respect to the central axis along the stacking direction.

7. The diaphragm is characterized in that the material contains silicon, as described in any one of claims 1 to 6, for the MEMS ultrasonic transducer.

8. The piezoelectric thin film is characterized in that it has legs with multiple ends extending toward the outer edge, as described in any one of claims 1 to 6.

9. The MEMS ultrasonic transducer according to claim 8, characterized in that the rigidity is adjusted by changing at least one of the number and width of the aforementioned legs.

10. A MEMS ultrasonic transducer according to any one of claims 1 to 6, further comprising a strain adjustment membrane provided in the gap of the diaphragm, which is a member for adjusting the magnitude of the initial strain.

11. A MEMS ultrasonic transducer according to any one of claims 1 to 6, It includes a measuring unit that measures the distance to the object being measured based on the time difference between the transmission and reception of ultrasonic waves, The measurement unit is characterized by transmitting the ultrasonic waves from the MEMS ultrasonic transducer at a transmission frequency lower than the resonant frequency, applying a DC voltage to the lower electrode to change the resonant frequency of the diaphragm to the transmission frequency, and receiving the ultrasonic waves reflected from the object to be measured.