A micro-electromechanical piezoelectric ultrasonic transducer with etched holes and segmented upper electrodes

Abstract

This invention discloses a microprocessor-based piezoelectric ultrasonic transducer with etched holes and a segmented upper electrode. The transducer comprises a substrate, a structural layer, a bottom electrode, a piezoelectric layer, and an upper electrode. The transducer operates by utilizing the direct and inverse piezoelectric effects to convert mechanical energy into electrical energy or vice versa. This invention divides the original square ring structure of the upper electrode and piezoelectric layer into four equal-area rectangular structures. This structure reduces the facing area between the upper and bottom electrodes, thereby reducing parasitic capacitance and improving the signal-to-noise ratio of the ultrasonic transducer. Simultaneously, small holes are etched at the four empty corners, which helps reduce the stiffness of the diaphragm and improve the transducer's emission sensitivity, thus compensating for the loss in emission sensitivity caused by reducing the area of ​​the upper electrode.

Description

Technical Field

[0001] The present invention belongs to the field of transducers in the technical field of micro-electromechanical systems (MEMS), and in particular relates to a piezoelectric ultrasonic transducer with high signal-to-noise ratio and high sensitivity. Background Art

[0002] Piezoelectric ultrasonic transducer is a device that can convert electrical energy into mechanical energy and mechanical energy into electrical energy. It is a device that can transmit and receive ultrasonic waves. Traditional ultrasonic transducers are processed by mechanical processing, which is large in size, high in power consumption, and not conducive to integration. In addition, since its acoustic impedance does not match the traditional sound transmission medium (air, water), its acoustic emission efficiency is low. The micro-manufacturing process combined with micro-electromechanical system technology effectively overcomes the above shortcomings. According to their working principles, MEMS ultrasonic transducers can be mainly divided into MEMS capacitive ultrasonic transducers and MEMS piezoelectric ultrasonic transducers: the sensitivity and bandwidth of capacitive ultrasonic transducers have certain advantages over piezoelectric ultrasonic transducers, but they require very high DC bias voltage and extremely narrow capacitor gap to achieve, and are limited by limited amplitude, so the sound pressure they emit is also limited [1]. On the contrary, the amplitude of piezoelectric ultrasonic transducers is much larger, but due to the limited piezoelectric constant and inherent parasitic capacitance of piezoelectric materials at this stage, the signal-to-noise ratio and sensitivity of MEMS piezoelectric ultrasonic transducers need to be further optimized. The MEMS piezoelectric ultrasonic transducer with etched holes and block-type upper electrodes designed in the present invention can effectively improve its sensitivity and increase its signal-to-noise ratio, and has greater advantages in daily applications such as fingerprint recognition [2], distance detection [3] and energy harvesting [4].

[0003] References:

[0004] [1]Jung J,Kim S,Lee W and Choi H 2013Fabrication of a two-dimensionalpiezoelectric micromachined ultrasonic transducer array using a top-crossover-to-bottom structure and metal bridge connections,J.MicromechanicsMicroengineering 23 125037.

[0005] [2]Przybyla RJ, Tang HY, Shelton SE, Horsley DA and Boser BE 201412.1 3D ultrasonic recognition gesture Dig.Tech.Pap.-IEEE Int.Solid-StateCircuits Conf.57 210–1.

[0006] [3]Przybyla RJ,Tang H,Member S,Guedes A,Shelton SE,Horsley DA andBoser BE 2015 3D Ultrasonic Range finder on a Chip IEEE J.Solid-StateCircuits 50 320–34

[0007] [4]He Q, Liu J, Yang B, Wang X, Chen X and Yang C 2014 MEMS-based ultrasonic transducer as the receiver for wireless power supply of theimplantable microdevices Sensors Actuators,A Phys.219 65–72 Summary of the invention

[0008] The purpose of the present invention is to solve the problems of low signal-to-noise ratio and low sensitivity in the prior art, and to provide a micro-electromechanical piezoelectric ultrasonic transducer with etched holes and a block-type upper electrode.

[0009] The technical solution adopted by the present invention to solve its technical problem is:

[0010] A micro-electromechanical piezoelectric ultrasonic transducer with an etched hole and a block-type upper electrode, characterized in that it comprises a substrate, a bottom electrode, a piezoelectric layer and an upper electrode, a groove-shaped cavity is provided in the middle of the back side of the substrate, so that an elastic structure layer (41) is formed on the front side of the substrate, and the substrate around the groove-shaped cavity serves as a fixed end of the elastic structure layer; the front side of the elastic structure layer is stacked with the bottom electrode, the piezoelectric layer and the upper electrode in sequence, and together with the elastic structure layer, a vibrating film is formed; wherein the bottom electrode covers the front side of the substrate, and the piezoelectric layer is composed of four rectangular units of equal area, which are centrally symmetrically distributed along the center point of the elastic structure layer to form a square ring with missing corners, and the four rectangular units are not in contact with each other; the structure of the upper electrode is completely the same as that of the piezoelectric layer, and the four rectangular units of the upper electrode overlap with the four rectangular units of the piezoelectric layer in a one-to-one correspondence; at the four missing corners of the square ring, an array of through holes penetrating the bottom electrode and the elastic structure layer is formed by etching; and neither the piezoelectric layer nor the upper electrode covers the small holes.

[0011] When the transducer is used as a transmitter, an alternating voltage is applied to the upper and bottom electrodes to drive the diaphragm to vibrate out of the plane using the inverse piezoelectric effect, thereby generating ultrasonic waves; when used as a receiver, the direct piezoelectric effect is used to convert the diaphragm vibration caused by external ultrasonic excitation into an electrical signal output.

[0012] Preferably, in the through hole array, the aperture (diameter) of the through holes is 3-5 μm, and the spacing is 6-8 μm.

[0013] Preferably, the width of the four rectangular units of the upper electrode and the four rectangular units of the piezoelectric layer are 30%-35% of the half length of the groove-shaped cavity, and the length is 65%-70% of the length of the groove-shaped cavity; the outer edge of the square ring coincides with the outer edge of the groove-shaped cavity.

[0014] Preferably, the depth of the through hole is consistent with the sum of the thickness of the elastic structure layer and the bottom electrode, which is 4 to 7 μm.

[0015] Preferably, the substrate and the elastic structure layer are made of semiconductor material silicon, including single crystal silicon or polycrystalline silicon.

[0016] Preferably, the groove-shaped cavity is formed by selective etching on the back side of the substrate.

[0017] Preferably, the piezoelectric material in the piezoelectric layer is aluminum nitride, zinc oxide or lead zirconate titanate piezoelectric ceramic.

[0018] Preferably, the bottom electrode is boron-doped silicon, or a metal material such as gold, platinum, aluminum or tin.

[0019] Preferably, the upper electrode is made of metal materials such as gold, platinum, aluminum or tin.

[0020] Preferably, the four rectangular units of the upper electrode are connected to the same excitation source, and the upper electrode is excited by the same excitation source.

[0021] The working principle of the transducer of the present invention is to realize the conversion of mechanical energy into electrical energy or electrical energy into mechanical energy by using the positive and negative piezoelectric effects. The upper electrode of the original square ring is cut into four rectangular blocks of equal area, so as to change the corresponding area of ​​the upper and lower electrodes of the transducer, thereby reducing the parasitic capacitance value, which is conducive to improving the signal-to-noise ratio of the transducer. At the same time, small holes are etched at the four corners of the original square ring, which is conducive to reducing the rigidity of the diaphragm, thereby improving the sensitivity of the transducer. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] The present invention is further described below with reference to the accompanying drawings and embodiments.

[0023] Figure 1 It is a schematic structural diagram of a micro-electromechanical piezoelectric ultrasonic transducer with an etched hole and a block-type upper electrode in the present invention;

[0024] Figure 2 yes Figure 1 AA cross-sectional diagram of ;

[0025] Figure 3 yes Figure 2 A partial enlarged view of the dotted frame B;

[0026] Figure 4 It is an equivalent circuit diagram of a micro-machine piezoelectric ultrasonic transducer with an etched hole and a block-type upper electrode;

[0027] In the figure: upper electrode 1, piezoelectric layer 2, bottom electrode 3, substrate 4, elastic structure layer 41, vibration film 00. DETAILED DESCRIPTION

[0028] The present invention is further described and illustrated below in conjunction with the accompanying drawings and specific embodiments. The technical features of each embodiment of the present invention can be combined accordingly without conflicting with each other.

[0029] like Figures 1 to 3As shown, a micro-electromechanical piezoelectric ultrasonic transducer with an etched hole and a block-type upper electrode mainly includes a substrate 4, a bottom electrode 3, a piezoelectric layer 2 and an upper electrode 1. By selectively etching a groove-shaped cavity on the back of the substrate 4, an elastic structure layer 41 of a certain thickness is formed on the front of the substrate 4, and the part of the substrate 4 that is not etched around the elastic structure layer 41 forms a fixed end (anchor end). The front of the elastic structure layer 41 is stacked with a bottom electrode 3, a piezoelectric layer 2 and an upper electrode 1 from bottom to top, and the bottom electrode 3, the piezoelectric layer 2 and the upper electrode 1 together with the elastic structure layer 41 constitute a vibrating film 00. Among them, the bottom electrode 3 covers the upper surface of the substrate 4, while the coverage area of ​​the piezoelectric layer 2 and the upper electrode 1 is relatively small. The piezoelectric layer 2 is composed of 4 rectangular units of equal area, which are centrally symmetrically distributed along the center point of the elastic structure layer 41. The four rectangular units of the piezoelectric layer 2 are each close to an outer edge of the elastic structure layer 41, forming an approximate square ring, but the four rectangular units do not touch each other, so that a square area is missing at the four corners of the square ring. But overall, the outer edge of the square ring coincides with the projection of the outer edge of the slot-shaped cavity. Similarly, the structure of the upper electrode 1 is exactly the same as that of the piezoelectric layer 2. The upper electrode 1 is also divided into four rectangular units of equal area on the basis of the original square ring to form a block-type upper electrode. The four rectangular units of the upper electrode 1 overlap and cover the four rectangular units of the piezoelectric layer 2 one by one. In the four missing corner areas of the square ring of the piezoelectric layer 2 and the upper electrode 1, an N×N through-hole array is etched, and each through-hole penetrates the elastic structure layer 41 and the bottom electrode 3 during the etching process. The four rectangular units of the upper electrode are connected to the same excitation source and are excited by the same excitation source. As Figure 1 As shown, the upper electrode 1 and the piezoelectric layer 2 are evenly distributed on the four sides, and the piezoelectric layer 2 and the upper electrode 1 are adjacent to the small hole and do not cover the small hole. When the transducer is used as a transmitter, the inverse piezoelectric effect is used to drive the diaphragm to vibrate out of the plane by applying alternating voltages to the upper and bottom electrodes, thereby generating ultrasonic waves; when used as a receiver, the positive piezoelectric effect is used to convert the diaphragm vibration caused by external ultrasonic excitation into an electrical signal output.

[0030] In addition, in this embodiment, the structural parameters of each component are as follows: the material of the upper electrode 1 is an aluminum electrode with a thickness of 1 μm; the material of the piezoelectric layer 2 is aluminum nitride with a thickness of 0.5 μm. Any rectangular unit of the upper electrode 1 and any rectangular unit of the piezoelectric layer 2 have a width of 37.5 μm and a length of 175 μm. Figures 1 to 3In order to distinguish the upper electrode and the piezoelectric layer, the distance between the inner and outer edges of the piezoelectric layer and the center of the elastic structure layer is appropriately extended. However, in this embodiment, the inner and outer edges of the upper electrode and the piezoelectric layer are actually completely overlapped in the vertical direction (i.e., horizontal projection), and the square ring formed by the upper electrode 1 and the square ring formed by the piezoelectric layer 2 are also completely overlapped with the outer edge of the slot-shaped cavity in the horizontal projection. The bottom electrode 3 is made of boron-doped silicon with a thickness of 1μm. The substrate 4 is made of semiconductor silicon with a thickness of 400μm. The diaphragm 00 is released by selectively etching part of the silicon from the back. The thickness of the elastic structure layer 41 formed after etching is 5μm, and the length and width are 250μm. The depth of the small hole is consistent with the sum of the thickness of the elastic structure layer 41 and the bottom electrode 3, which is 6μm. In each through-hole array in the missing corner area, the array is a 4×4 rectangular array, the aperture of each through-hole is 3-5μm, and the spacing between adjacent holes in the array is the same, which is 6-8μm.

[0031] like Figure 4 As shown in the equivalent circuit diagram of the above transducer, there is a parasitic capacitor C0 between the upper electrode 1 and the bottom electrode 3, which brings noise to the circuit and reduces the performance of the transducer. According to the formula It can be seen that reducing the relative area S of the two plates can reduce the parasitic capacitance value accordingly and improve the signal-to-noise ratio of the transducer. In addition, small holes are etched in the bottom electrode and the structural layer to reduce the stiffness of the diaphragm and improve the sensitivity of the transducer.

[0032] The present invention proposes a micro-electromechanical voltage-electric ultrasonic transducer with an etched hole and a block-type upper electrode. This structure, which is formed by cutting into a block-type upper electrode and etching small holes at the corners, can reduce the area directly facing the upper electrode and the bottom electrode, thereby reducing the harm of parasitic capacitance and improving the signal-to-noise ratio of the transducer. At the same time, due to the structure of the small holes, the stiffness of the diaphragm is reduced, further improving the sensitivity of the transducer. In summary, the micro-electromechanical voltage-electric ultrasonic transducer with an etched hole and a block-type upper electrode has the advantages of high signal-to-noise ratio and high sensitivity.

Claims

1. A micro-electromechanical piezoelectric ultrasonic transducer having an etched hole and a block-type upper electrode, characterized in that: The invention comprises a substrate (4), a bottom electrode (3), a piezoelectric layer (2) and an upper electrode (1); a groove-shaped cavity is provided in the middle of the back side of the substrate (4), so that an elastic structure layer (41) is formed on the front side of the substrate (4); the substrate (4) around the groove-shaped cavity serves as a fixed end of the elastic structure layer (41); the front side of the elastic structure layer (41) is stacked with a bottom electrode (3), a piezoelectric layer (2) and an upper electrode (1) in sequence, and together with the elastic structure layer (41) forms a vibrating film (00); wherein the bottom electrode (3) covers the front side of the substrate (4), and the piezoelectric layer (2) The invention is composed of four rectangular units of equal area, which are centrally symmetrically distributed along the center point of the elastic structure layer (41) to form a square ring with missing corners, and the four rectangular units do not contact each other; the structure of the upper electrode (1) is exactly the same as that of the piezoelectric layer (2), and the four rectangular units of the upper electrode (1) overlap with the four rectangular units of the piezoelectric layer (2) in a one-to-one correspondence; at the four missing corners of the square ring, an array of through holes penetrating the bottom electrode (3) and the elastic structure layer (41) is etched; and the piezoelectric layer (2) and the upper electrode (1) do not cover the small holes; In the through hole array, the aperture of the through hole is 3-5 μm, and the spacing is 6-8 μm; the depth of the through hole is consistent with the sum of the thickness of the elastic structure layer (41) and the bottom electrode (3), which is 4-7 μm; The width of the four rectangular units of the upper electrode (1) and the four rectangular units of the piezoelectric layer (2) are both 30%-35% of the half length of the groove-shaped cavity, and the length is 65%-70% of the length of the groove-shaped cavity; the outer edge of the square ring coincides with the outer edge of the groove-shaped cavity.

2. The micro-electromechanical piezoelectric ultrasonic transducer with etched holes and segmented upper electrodes according to claim 1, characterized in that: The substrate (4) and the elastic structure layer (41) are made of semiconductor material silicon, including single crystal silicon or polycrystalline silicon.

3. The micro-electromechanical piezoelectric ultrasonic transducer with etched holes and segmented upper electrodes according to claim 1, characterized in that: The groove-shaped cavity is formed by selectively etching the back side of the substrate (4).

4. The micro-electromechanical piezoelectric ultrasonic transducer with etched holes and segmented upper electrodes according to claim 1, characterized in that: The piezoelectric material in the piezoelectric layer (2) is aluminum nitride, zinc oxide or lead zirconate titanate piezoelectric ceramic.

5. The micro-electromechanical piezoelectric ultrasonic transducer with etched holes and segmented upper electrodes according to claim 1, characterized in that: The bottom electrode (3) is boron-doped silicon, or a metal material such as gold, platinum, aluminum or tin.

6. The micro-electromechanical piezoelectric ultrasonic transducer with etched holes and segmented upper electrodes according to claim 1, characterized in that: The upper electrode (1) is made of a metal material such as gold, platinum, aluminum or tin.

7. The micro-electromechanical piezoelectric ultrasonic transducer with etched holes and segmented upper electrodes according to claim 1, characterized in that: The four rectangular units of the upper electrode (1) are connected to the same excitation source.

Images