Multi-electrode driven high-order mode piezoelectric micromachined ultrasonic transducer and its application

Abstract

One technical solution of the present invention is to provide a multi-electrode driven high-order mode piezoelectric micromechanical ultrasonic transducer. Another technical solution of the present invention is to provide an application of the aforementioned multi-electrode driven high-order mode piezoelectric micromechanical ultrasonic transducer. The present invention provides a multi-electrode driven piezoelectric micromechanical ultrasonic transducer based on high-order modes. Under the preferred driving electrode design, the emission sensitivity of the n-order mode is approximately n times the sound pressure of the first-order mode. 2 The high-order mode piezoelectric micromachined ultrasonic transducer solves the problems of low emission sound pressure, low emission efficiency, and poor anti-crosstalk capability of the first-order mode piezoelectric micromachined ultrasonic transducer. Therefore, the high-order mode piezoelectric micromachined ultrasonic transducer has significant advantages in forming large-scale, highly directional arrays.

Description

Technical Field

[0001] The invention relates to a piezoelectric micromechanical ultrasonic transducer driven by multiple electrodes based on high-order modes. Background Art

[0002] Piezoelectric micromachined ultrasonic transducers (PMUTs) are MEMS devices that transmit and receive ultrasonic waves by vibrating a piezoelectric film through the inverse and direct piezoelectric effects. They are widely used in fields such as fingerprint recognition, target detection, and medical imaging [1-3]. Ultrasonic transducers based on piezoelectric ceramic materials have a significant acoustic impedance mismatch compared to the acoustic impedance of air or liquid, and the cutting technology required to process them into two-dimensional arrays is expensive. Compared to ultrasonic transducers based on piezoelectric materials, piezoelectric micromachined ultrasonic transducers have the advantages of easy acoustic impedance matching with air or liquid, low power consumption, low cost (using planar processing technology), and ease of integration with peripheral circuits.

[0003] Traditional piezoelectric micromachined ultrasonic transducers are typically based on a first-order resonant mode (also known as the fundamental mode). First-order piezoelectric micromachined ultrasonic transducers have a low frequency, and the emitted sound pressure is proportional to the square of the frequency, so the emitted sound pressure in the first-order mode is also low. Furthermore, first-order piezoelectric micromachined ultrasonic transducers have poor immunity to external interference and are susceptible to crosstalk from adjacent units. When formed into large-scale arrays, it is difficult to form a concentrated beam, which reduces the emitted sound pressure.

[0004] [1]X.Jiang,Y.Lu,H.-Y.Tang,JMTsai,EJNg,MJDaneman,BEBoser,andD.A.Horsley, "Monolithic ultrasound fingerprint sensor,"Microsystems&Nanoengineering,vol.3,no.1,pp.1–8,Nov.2017.

[0005] [2] DEDausch, KHGilchrist, JB Carlson, SDHall, JBCastellucci, and O.T. von Ramm, "In vivo real-time 3-D intracardiac echo using PMUT arrays," IEEE transactions on ultrasonics, ferroelectrics, and frequency control, vol.61, no.10, pp.1754–1764, Oct.2014.

[0006] [3]Y.Lu, A.Heidari, and DAHorsley, "A High Fill-Factor Annular Array of High Frequency Piezoelectric Micromachined Ultrasonic Transducers," Journal of Microelectromechanical Systems, vol.24, no.4, pp.904–913, 2015. Summary of the Invention

[0007] The technical problem to be solved by the present invention is that the emission sound pressure of the first-order mode piezoelectric micromechanical ultrasonic transducer is low, the emission efficiency is low, and the anti-interference ability is poor.

[0008] In order to solve the above technical problems, a technical solution of the present invention is to provide a multi-electrode driven high-order mode piezoelectric micromachined ultrasonic transducer, characterized in that it includes a multi-layer suspended membrane structure. For a multi-layer suspended membrane structure adopting a single piezoelectric layer structure, the multi-layer suspended membrane structure includes at least a piezoelectric layer, an elastic layer and an electrode layer, wherein:

[0009] The piezoelectric layer is a piezoelectric material layer;

[0010] The elastic layer is a non-piezoelectric material stack, or a piezoelectric material stack without voltage drive;

[0011] The electrode layer is used to form a multi-layer electrode group, with one electrode group on both the upper and lower sides of the piezoelectric layer;

[0012] When an external electric field is applied, stress / strain is generated in the piezoelectric layer between the upper and lower electrode groups, while the elastic layer is unaffected by the electric field. The difference in stress / strain between the piezoelectric and elastic layers causes the multilayer suspended membrane structure to bend.

[0013] For a multi-layer suspended membrane structure using n piezoelectric layers, n≥2, the multi-layer suspended membrane structure includes at least n piezoelectric layers, an elastic layer, and an electrode layer, wherein:

[0014] The piezoelectric layer is a piezoelectric material layer;

[0015] The elastic layer is a non-piezoelectric material stack, or a piezoelectric material stack without voltage drive;

[0016] Electrode layers are used to form a multi-layer electrode group, with one electrode group on both the upper and lower sides of each piezoelectric layer;

[0017] When an external electric field is applied, the piezoelectric layer between the upper and lower electrode groups generates stress / strain, and the stress / strain generated by adjacent piezoelectric layers is opposite. The difference in stress / strain between two adjacent piezoelectric layers causes the multilayer suspended membrane structure to bend.

[0018] In the present invention, the relative positions of the stacked layers can be changed without affecting the acoustic wave generation effect of the device. For example, the elastic layer can be above or below the piezoelectric layer without significantly changing the acoustic wave generation effect.

[0019] When the electrodes in the electrode group make the stress distribution generated by the piezoelectric layer more consistent with the stress distribution of a certain characteristic mode, it is more conducive to exciting that characteristic mode. The present invention uses multi-electrode excitation to effectively excite higher-order modes. In certain characteristic modes, there is a diameter where the stress zero crossing passes through the multi-layer suspended membrane structure, which is called a node diameter. If the stress zero crossing forms a ring, it is called a node ring.

[0020] Preferably, the gap between the electrodes in the electrode group is a node diameter or a node ring formed by the stress zero-crossing point of the electrode layer.

[0021] Preferably, when the number of node diameters is fixed, the higher the symmetrical mode is, the higher the frequency is, and the more conducive it is to generating a high emission sound pressure. The sound pressure emitted by the axisymmetric n-order mode is approximately n times the sound pressure of the first-order mode. 2 times.

[0022] Preferably, the electrode groups are independently controlled, or connected in parallel to voltage ports of the same polarity according to the polarity of the stress.

[0023] Preferably, the piezoelectric material layer is a single-layer structure or a stacked-layer structure.

[0024] Preferably, the thickness of the piezoelectric layer or the elastic layer is between 1-100 um; the thickness of the electrode layer is between 1 nm-10 um.

[0025] Preferably, the multi-layer suspended membrane structure is circular, elliptical or polygonal.

[0026] Another technical solution of the present invention is to provide an application of the aforementioned multi-electrode driven high-order mode piezoelectric micromechanical ultrasonic transducer, characterized in that a plurality of the aforementioned multi-electrode driven high-order mode piezoelectric micromechanical ultrasonic transducers form a one-dimensional or two-dimensional array.

[0027] Preferably, the one-dimensional or two-dimensional array is a linear array, a square array, a circular array or a spiral array.

[0028] Compared with the existing technical solutions, the present invention has the following beneficial effects:

[0029] The present invention provides a piezoelectric micromechanical ultrasonic transducer based on a multi-electrode drive and a high-order mode. Under the preferred driving electrode design, the transmission sensitivity of the n-order mode is approximately n times the sound pressure of the first-order mode. 2 The high-order mode piezoelectric micromachined ultrasonic transducer solves the problems of low emission sound pressure, low emission efficiency, and poor anti-crosstalk capability of the first-order piezoelectric micromachined ultrasonic transducer. Therefore, the high-order mode piezoelectric micromachined ultrasonic transducer has significant advantages in forming large-scale, highly directional arrays. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 Schematic diagram of the cross section of a multi-electrode driven piezoelectric micromachined ultrasonic transducer;

[0031] Figure 2 Top view of a circular multi-electrode driven piezoelectric micromachined ultrasonic transducer;

[0032] Figure 3a 、 Figure 3b 、 Figure 3c The three-dimensional views of the deformation and stress distribution of the first, third, and fifth-order axisymmetric modes are shown. In the figure, light colors indicate negative stress values, and the lighter the color, the larger the value; dark colors indicate positive stress values, and the darker the color, the larger the value;

[0033] Figure 4a 、 Figure 4b 、 Figure 4c These are the deformation and stress distribution cross-sections of the first, third, and fifth-order axisymmetric modes. In the figure: light colors indicate negative stress values, and the lighter the color, the larger the value; dark colors indicate positive stress values, and the darker the color, the larger the value.

[0034] Figure 5 The conventional first-order mode piezoelectric micromachined ultrasonic transducer and the preferred electrode configuration are shown. In the figure, electrode 1 is connected to an AC potential, and electrode 3 is connected to zero potential. The coverage of electrode 1 is about 70%.

[0035] Figure 6The diagram shows a third-order mode piezoelectric micromachined ultrasonic transducer and its preferred electrode configuration. In the figure, electrode 1a is connected to a positive polarity AC potential, electrode 1b is connected to an AC potential with an anti-phase phase difference of 1a (180°); electrode 3 is connected to zero potential.

[0036] Figure 7 The figure shows the comparison of the transmission, reception, and round-trip sensitivity of Comparative Example 1 and Comparative Example 2 immersed in water at the same cavity radius using finite element simulation, where the transmission sensitivity is defined as the ratio of the sound pressure at a distance of 2 mm from the surface of the piezoelectric micromechanical ultrasonic transducer to the driving voltage amplitude, the reception sensitivity is the ratio of the receiving voltage to the sound pressure of the sound source at a distance of 2 mm, and the round-trip sensitivity is the product of the transmission sensitivity and the reception sensitivity.

[0037] Figure 8 The directional comparison between Comparative Example 1 and Comparative Example 2 is shown;

[0038] Figure 9 The sound pressure-frequency response at 2 mm when Comparative Example 1 and Comparative Example 2 are respectively formed into a 3×3 array is shown;

[0039] Figure 10 Schematic diagram of the cross section of a piezoelectric micromachined ultrasonic transducer driven by multiple electrodes using dual piezoelectric layers. DETAILED DESCRIPTION

[0040] Below in conjunction with specific embodiment, further set forth the present invention.Should be understood that these embodiments are only used to illustrate the present invention and are not used in limiting the scope of the present invention.In addition, should be understood that after reading the content taught by the present invention, those skilled in the art can make various changes or modifications to the present invention, and these equivalent forms fall equally within the scope limited by the appended claims of the application.

[0041] Example 1

[0042] Figure 1 A cross-sectional schematic diagram of an embodiment of a single piezoelectric stack of a multi-electrode driven piezoelectric micromachined ultrasonic transducer (PMUT) provided in the first embodiment is shown, wherein the thin film stack is suspended above the cavity 7 .

[0043] The cavity 7 can be formed by various processes, including but not limited to: backside deep reactive ion etching, pre-cavity silicon-on-insulator (cavity-SOI) technology, and pre-depositing a sacrificial layer and then etching the sacrificial layer 5 from the front side. Depending on the process, the sacrificial layer 5 and the sacrificial layer 6 can be made of the same or different materials.

[0044] The thin film stack includes an elastic layer 4, a lower electrode group 3a / 3b, a piezoelectric layer 2, and an upper electrode group 1a / 1b. The piezoelectric layer 2 can comprise one or more piezoelectric materials, such as zinc oxide, aluminum nitride, scandium-doped aluminum nitride, lead zirconate titanate, lithium niobate, lithium tantalate, PVDF, or other piezoelectric materials or piezoelectric material stacks. The electrodes are made of one or more conductive materials, including various metals (e.g., gold, copper, aluminum, silver), alloys, conductive compounds (e.g., indium tin oxide), and organic materials (e.g., conductive plastics), to form a multilayer electrode group. Electrode group 1a / 3a is connected to an AC voltage, while electrode group 1b / 3b is connected to an AC voltage that is anti-phase (180° out of phase) with electrode group 1a / 3a. Consequently, the stress generated in the piezoelectric layer 2 due to the piezoelectric effect between electrode groups 1a and 3b is of opposite polarity to the stress between electrode groups 1b and 3a, causing the thin film stack to distort due to the stress. When the frequency of the stress, that is, the frequency of the input AC voltage, is close to a certain characteristic mode frequency, the characteristic frequency is excited. There are gaps between the electrode groups 1a / 1b or 3a / 3b to ensure independent electrical connections.

[0045] In a preferred embodiment, the gaps between electrode groups 1a / 1b or 3a / 3b should be aligned with points where stress in the film stack is zero. For a circular, axisymmetric pattern, these points of zero stress form rings, so the area where the electrodes are placed is also divided into rings. Generally, adjacent electrodes have opposite potentials, meaning that electrode groups 1a and 1b are connected alternately to these rings, and similarly, electrode groups 3a and 3b.

[0046] For ease of description, the electrical connection of electrode group 1a / 1b is discussed here, which also applies to electrode group 3a / 3b. The electrodes within electrode group 1a or 1b can be independent or connected to the same potential if they are independently connected. In order to excite the characteristic mode with maximum efficiency, a preferred embodiment is that the voltage on the electrodes within electrode group 1a or 1b obeys the zero-order Bessel function distribution. In order to reduce the complexity of the electrical connection during use, the two electrode groups 1a / 1b are usually connected to voltages of two polarities respectively.

[0047] To further reduce process and electrical connection complexity, electrode groups 3a / 3b can be combined into a single electrode, while retaining the independence of electrode groups 1a / 1b. If electrode groups 1a / 1b are connected to two voltage terminals of different polarities, a three-terminal component is formed. If the independence of each electrode is retained, a multi-terminal component is formed.

[0048] like Figure 1 The embodiment shown can be easily extended to piezoelectric micromachined ultrasonic transducers with higher-order modes and more electrodes. The shape of the piezoelectric micromachined ultrasonic transducer can be polygonal, circular, elliptical, etc., and the shape of the driving electrode can also be block-shaped or ring-shaped.

[0049] Figure 2 A top view of an embodiment is shown, where the interconnection between the driving electrode and the peripheral circuit is not shown. In actual application, the annular driving electrode needs to have a gap to achieve interconnection with the peripheral circuit.

[0050] Generally, the effective displacement coefficient of the even-order symmetric mode is negative, that is, the sound pressure is emitted in the opposite direction, while the odd-order mode emits the sound pressure perpendicular to the plane, so the odd-order mode is preferred.

[0051] Figure 3a 、 Figure 3b as well as Figure 3c (hereinafter referred to as "Figure 3") shows Figure 2 3D views of deformation and stress distribution for the first, third, and fifth symmetric modes in a circular embodiment. Figure 4a 、 Figure 4b as well as Figure 4c (hereinafter collectively referred to as "Figure 4") shows the cross-sectional diagram of the deformation and stress distribution of the first, third, and fifth order symmetric modes. In Figures 3 and 4: light colors indicate negative stress values, and the lighter the color, the smaller the value; dark colors indicate positive stress values, and the darker the color, the larger the value. There is no voltage acting on the surrounding substrate, and the stress is zero. It can be found that the stress on the upper and lower surfaces is always of opposite polarity; the stress between adjacent regions is usually also of opposite polarity, and there is a point where the stress crosses zero between adjacent regions. For Figure 2 In the circular embodiment shown, the stress zero crossings form a ring. Using the stress zero crossing lines to separate the electrodes yields an optimal electrode arrangement for stimulating this characteristic mode. A preferred embodiment is that if the electrodes are independent, the voltages across them should obey a zero-order Bessel function distribution. Given the complexity of practical electrical connections, electrodes of the same polarity are typically connected to voltages of the same polarity.

[0052] To reduce redundancy, the third-order mode embodiment is used as an example to compare with the first-order embodiment. These improvements are also applicable to piezoelectric micromachined ultrasonic transducers with higher-order modes.

[0053] Figure 5 The conventional first-order mode and its preferred electrode configuration are shown, referred to as "Comparative Example 1". Electrode 1 is connected to an AC potential, and electrode 3 is connected to zero potential. The coverage of electrode 1 is approximately 70%. Figure 6 The third-order mode and its preferred electrode configuration are shown, which is called "Comparative Example 2". Electrode 1a is connected to a positive polarity AC potential, electrode 1b is connected to an AC potential with an anti-phase (180° phase difference) with 1a; and electrode 3 is connected to zero potential.

[0054] Figure 7This figure shows a comparison of the transmit, receive, and round-trip sensitivity of Comparative Examples 1 and 2, using finite element simulations of the same cavity radius, immersed in water. Transmit sensitivity is defined as the ratio of the sound pressure at a distance of 2 mm from the piezoelectric micromachined ultrasonic transducer surface to the driving voltage amplitude (in Pa / V). Receive sensitivity is the ratio of the received voltage to the sound pressure of the plane wave source at a distance of 2 mm (in V / Pa). Round-trip sensitivity is the product of transmit sensitivity and receive sensitivity. Figure 7 It shows that the transmission sensitivity of the comparative embodiment 2 (third-order mode) is 11.5 times that of the comparative embodiment 1 (first-order mode), and the round-trip sensitivity of the comparative embodiment 2 is 3.6 times that of the comparative embodiment 1.

[0055] Figure 8 The directional comparison between Comparative Example 1 and Comparative Example 2 in finite element simulation is shown. Figure 8 It is shown that the comparative example 2 has higher directivity than the comparative example 1, that is, a more concentrated sound field, and the sound field is more concentrated on the center line of the piezoelectric micromechanical ultrasonic transducer. The first-order mode represented by the comparative example 1 is almost omnidirectional, that is, the radiation energy is equal on the spherical surface, which means that the energy is dissipated very quickly, and it also causes the energy emitted by the nearby piezoelectric micromechanical ultrasonic transducer received by the piezoelectric micromechanical ultrasonic transducer to cause the resonant mode to be disturbed (also known as crosstalk). Such characteristics make it difficult for the comparative example 1 to form a concentrated and stable beam when forming a large-scale array, which is not conducive to target recognition. On the contrary, the comparative example 2 has high directivity and concentrated energy, which is conducive to the formation of a large-scale array.

[0056] Figure 9 The sound pressure frequency response of Comparative Examples 1 and 2, respectively, at a distance of 2 mm from the axisymmetric centerline of the piezoelectric micromachined ultrasonic transducers, is shown for a 3×3 array. The first-order mode produces a chaotic frequency response due to mutual interference between adjacent units. The third-order mode piezoelectric micromachined ultrasonic transducers experience less crosstalk, resulting in a more concentrated frequency response near the resonant frequency.

[0057] Example 2

[0058] Figure 10 The cross-sectional diagram of a piezoelectric micromachined ultrasonic transducer driven by multiple electrodes using dual piezoelectric layers disclosed in Example 2 is shown. In Example 2, Figure 1 The elastic layer 4 is not necessary and is therefore not included in the Figure 10. The piezoelectric layer 9 and the piezoelectric layer 2 can be the same or different piezoelectric materials. In order to bend the film, the piezoelectric layer 9 and the piezoelectric layer 2 should generate opposite stresses. If the piezoelectric polarities of the two are the same, the desired opposite stresses can be obtained by using opposite electric fields. In Example 2, there are three electrode groups 1a / 1b, 3a / 3b and 8a / 8b. The interconnection method within the group is consistent with that described in Example 1, and can be independent and connected in parallel. In order to reduce the complexity of interconnection in actual applications, the electrode groups 1a, 3a, and 8a can be connected to a voltage of the same polarity, while the electrode groups 1b, 3b, and 8b are connected to a voltage of opposite polarity (180° phase difference). To further reduce the complexity of interconnection, the electrode groups 1a / 1b, 8a / 8b are merged into the same electrode, which is suspended or connected to zero potential. The voltages of opposite polarity are maintained between 3a / 3b, which can also drive the piezoelectric micromechanical ultrasonic transducer to work, but the driving efficiency is reduced compared to the interconnection methods mentioned above.

Claims

1. A multi-electrode driven high-order mode piezoelectric micromechanical ultrasonic transducer, characterized in that: The invention comprises a multi-layer suspended membrane structure. For a multi-layer suspended membrane structure adopting a single piezoelectric layer structure, the multi-layer suspended membrane structure comprises at least a piezoelectric layer, an elastic layer and an electrode layer, wherein: The piezoelectric layer is a piezoelectric material layer; The elastic layer is a non-piezoelectric material stack, or a piezoelectric material stack without voltage drive; The electrode layer is used to form a multi-layer electrode group, with one electrode group on both the upper and lower sides of the piezoelectric layer; When an external electric field is applied, stress / strain is generated in the piezoelectric layer between the upper and lower electrode groups. At the same time, the elastic layer is not affected by the electric field. Due to the difference in stress between the piezoelectric layer and the elastic layer, the multilayer suspended membrane structure bends. For a multi-layer suspended membrane structure using n piezoelectric layers, n≥2, the multi-layer suspended membrane structure includes at least n piezoelectric layers, an elastic layer, and an electrode layer, wherein: The piezoelectric layer is a piezoelectric material layer; The elastic layer is a non-piezoelectric material stack, or a piezoelectric material stack without voltage drive; Electrode layers are used to form a multi-layer electrode group, with one electrode group on both the upper and lower sides of each piezoelectric layer; When an external electric field is applied, the piezoelectric layers between the upper and lower electrode groups generate stress / strain, with adjacent piezoelectric layers generating opposite stresses. Based on the difference in stress between adjacent piezoelectric layers, the multilayer suspended membrane structure bends. When the electrodes in the electrode group make the stress distribution generated by the piezoelectric layer more consistent with the stress distribution of the first-order symmetric mode, the third-order symmetric mode, or the fifth-order symmetric mode, it is more conducive to exciting the characteristic mode, and the high-order mode can be effectively excited through multi-electrode excitation.

2. The multi-electrode driven high-order mode piezoelectric micromachined ultrasonic transducer according to claim 1, characterized in that: The piezoelectric material layer is a single-layer structure or a stacked-layer structure.

3. The multi-electrode driven high-order mode piezoelectric micromachined ultrasonic transducer according to claim 1, characterized in that: The electrode groups are independently controlled or connected in parallel to voltage terminals of the same polarity according to the polarity of the stress.

4. An application of the multi-electrode driven high-order mode piezoelectric micromachined ultrasonic transducer according to claim 1, characterized in that: A plurality of multi-electrode driven high-order mode piezoelectric micromechanical ultrasonic transducers as claimed in claim 1 form a one-dimensional or two-dimensional array.

5. A use according to claim 4, characterized in that, The one-dimensional or two-dimensional array is a linear array, a square array, a circular array or a spiral array.

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