CN115106275A - Micro-mechanical ultrasonic transducer based on support column - Google Patents
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- B06B1/00—Methods or apparatus for generating mechanical vibrations of infrasonic, sonic, or ultrasonic frequency
- B06B1/02—Methods or apparatus for generating mechanical vibrations of infrasonic, sonic, or ultrasonic frequency making use of electrical energy
- B06B1/06—Methods or apparatus for generating mechanical vibrations of infrasonic, sonic, or ultrasonic frequency making use of electrical energy operating with piezoelectric effect or with electrostriction
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Abstract
The invention belongs to the technical field of micro-electro-mechanical systems, and particularly relates to a micro-mechanical ultrasonic transducer based on a supporting column. The invention relates to a micro-mechanical ultrasonic transducer, which is a planar array formed by splicing and extending basic units; the basic unit comprises a piezoelectric stack, a neutral layer, a support pillar and a substrate; the piezoelectric stack comprises an upper electrode layer, a piezoelectric layer and a bottom electrode layer; the neutral layer provides support for the piezoelectric lamination, and when the piezoelectric lamination is excited to generate strain, the strain neutral surface of the composite film is positioned in the neutral layer; the supporting columns provide support for the neutral layer, are discretely distributed columnar structures, form a high-filling-density transducer, and improve the effective area of the thin film vibration; the upper electrode layer and the bottom electrode layer form an electrode pair for applying and collecting an electric field, charges and voltage on the piezoelectric layer; static control is performed on the piezoelectric stack by applying a direct current bias voltage; by applying an alternating-current coupling signal, the piezoelectric stack is dynamically excited, and high-performance ultrasonic signal receiving and transmitting are realized.
Description
Technical Field
The invention belongs to the technical field of micro-electro-mechanical systems, and particularly relates to a micro-mechanical ultrasonic transducer.
Background
The ultrasonic imaging has the advantages of no ionization side effect, high sensitivity, real-time imaging, no damage to tissues, low cost and the like, and is widely applied to the fields of medical imaging, industrial nondestructive testing, Internet of things, intelligent sensing and the like. The ultrasonic transducer is a key module in the application, is responsible for converting an ultrasonic signal and an electric signal, and directly determines the imaging quality and the sensing precision by the sensitivity performance and the bandwidth of the ultrasonic transducer.
The micro-mechanical ultrasonic transducer is processed by adopting an MEMS (micro-electromechanical systems) process, is suitable for efficiently preparing an ultrasonic transducer array, has the potential to be integrated with a CMOS (complementary metal oxide semiconductor) chip, and is beneficial to improving the signal-to-noise ratio and reducing the volume and the cost of an ultrasonic system. The basic principle of the micro-mechanical ultrasonic transducer is that through an electrostatic force effect or a piezoelectric effect, an electric signal excitation is converted into a mechanical excitation (or vice versa), so that a thin film at the center is bent and vibrated, a sound guide medium is extruded outwards, and the receiving and sending of ultrasonic waves are realized.
The traditional structure of the micro-mechanical ultrasonic transducer is based on the bending vibration mode of a disc or a square disc with a completely fixed edge, and shows a vibration form of local bulge in the middle of a film when excited by an electric signal or extruded by a sound-conducting medium. The structural design has the characteristics that the thicker side wall forms a ring surface support, the distance between units is large, the support part cannot vibrate, and the effective area is limited. This makes the total area available for vibration limited, and the output sound pressure and reception sensitivity poor when forming a large-scale array. Therefore, the effective area of the membrane vibration is increased, the loss is reduced, and the potential of heterogeneous packaging with a CMOS integrated circuit is combined, so that the performance and process bottleneck of the existing micromechanical transducer are broken through, and the micromechanical ultrasonic transducer array with high sensitivity and high integration is realized.
Disclosure of Invention
The invention aims to provide a micro-mechanical ultrasonic transducer based on a supporting column, which is used for enlarging the effective area of a vibrating diaphragm, improving the array filling density, having high sound pressure output and CMOS heterogeneous integration capability and realizing high-performance ultrasonic signal receiving and transmitting.
The invention provides a micro-mechanical ultrasonic transducer based on a supporting column, which is a planar array formed by extending basic units, and specifically comprises the following components: an array formed by full parallel splicing of basic units, or a linear array element formed by parallel splicing of a plurality of basic units (several to hundreds or even ten thousands), and then extending the formed linear array, or a two-dimensional array formed by full parallel array extension; wherein the structure of the basic unit comprises a piezoelectric stack 1, a neutral layer 2, a support column 3 and a substrate 4; the piezoelectric stack comprises a thin upper electrode layer 1-1, a piezoelectric layer 1-2 and a bottom electrode layer 1-3 from top to bottom; the neutral layer 2 is arranged below the bottom electrode layers 1-3, and provides a supporting function for the piezoelectric stack 1, and when the piezoelectric stack 1 is excited to generate strain, a strain neutral plane of a composite film formed by the piezoelectric stack 1 and the neutral layer 2 is positioned in the neutral layer 2; the supporting column 3 is positioned below the neutral layer 2 and provides support for the neutral layer; the substrate 4 is positioned below the support column 3 and provides a fixed support for the whole structure;
the upper electrode layer 1-1 and the bottom electrode layer 1-3 of the piezoelectric lamination layer 1 form an electrode pair for applying and collecting an electric field, electric charges and voltage on the piezoelectric layer 1-2; wherein the piezoelectric stack 1 can be statically controlled by applying a dc bias voltage; dynamically exciting the piezoelectric stack 1 by applying an ac coupling signal;
the peripheral outline of the structure of the basic unit is a convex polygon, such as a triangle, a square or a hexagon, as shown in fig. 2; typical shapes are regular triangles, squares, regular hexagons, etc. Fig. 1 is a schematic representation of a hexahedral array micromachined ultrasonic transducer formed by the splicing of elementary cells in equilateral triangles.
Further, the basic units in various shapes can be formed by combining and splicing basic units in one shape to form a planar array structure; or the basic units with different shapes can be combined and spliced to form a planar array structure; the side lengths of the basic units in different shapes are matched with each other, so that the boundaries among the different units are spliced tightly;
furthermore, the basic units with various shapes can also be extended and arranged to form a line array by the basic units with a single shape; or the basic units with different shapes can be combined and spliced to form a linear array, wherein the side lengths are mutually matched (opposite) so that the boundaries between different units are closely spliced;
furthermore, the line arrays can also be subjected to periodic continuation to form a two-dimensional array structure. As shown in fig. 6.
In the present invention, the support columns 3 are preferably shaped as discrete columnar structures, i.e., discrete columnar structures are formed between the substrate 4 and the neutral layer 2 in one basic unit;
further, the supporting columns 3 are discrete columnar structures inside the basic units, and may be strip-shaped frame structures (also called side walls) at the boundaries of the array formed by the combination of the basic units, as shown in fig. 4.
In the invention, when a fully parallel transducer or a parallel structure of array elements in a linear array transducer is formed, the supporting column 3 comprises a columnar (for example, cylindrical or square columnar) microstructure array with the same height and a side wall defining the outermost side boundary of the array. The inner boundary of the side wall is a closed figure, the side wall wraps a plurality of basic units of the transducer, and the outer part of the side wall continuously extends to another array or a cutting plane of the transducer chip. Because the side wall and the neutral plane have larger contact area, the side wall can provide higher-strength support for the vibration film so as to improve the reliability of the long-time work of the transducer. Furthermore, a fixed support of the partial boundaries is achieved, having a joint resonance mode across the cell when vibrating. The effective area of the membrane vibration can be increased and the inert area providing the fixed support is reduced, thus realizing a transducer with high packing density. The support post of the present invention further comprises conductive features to enable heterogeneous integration with the ultrasound front end chip and electrical control functions.
A flat contact surface is formed between the neutral layer 2 and the support pillar 3, and the neutral layer and the support pillar are combined through a bonding process;
in the present invention, in order to form a package by a process such as flip-chip mounting on a CMOS substrate, the positions of the piezoelectric stack 1 and the neutral layer 2 of the transducer are switched so that the neutral layer 2 is provided above the piezoelectric stack 1 and the supporting posts 3 are provided below the piezoelectric stack.
In the present invention, the substrate 4 may be a bare wafer without circuit function (e.g., a silicon wafer, a silicon-on-insulator (SOI) wafer, a borosilicate glass wafer), or a wafer with circuit function (e.g., a wafer on which a Complementary Metal Oxide Semiconductor (CMOS) circuit is formed).
When the substrate 4 is a bare wafer without circuit function, the upper support posts 3 are formed by etching the bare wafer through a top-down patterning process, and form an integrated structure with the substrate 4, or form a columnar microstructure above the substrate 4 by etching or stripping an original photoresist pattern through a bottom-up deposition process.
When the substrate 4 is a wafer having a circuit function, the upper support pillars 3 have conductivity, one end thereof is in contact with the upper electrode layer 1-1, or the bottom electrode layer 1-3 exposed by etching, and the other end thereof is connected to an electrode on the surface of the substrate 4. The original photoresist pattern is etched or stripped by a bottom-up deposition process to form a columnar microstructure above the substrate 4, which can be further controlled by a circuit.
In the invention, the electrostatic force effect is introduced to work together with the piezoelectric effect, so that the sensitivity of the micro-mechanical ultrasonic transducer can be further enhanced. Namely, at the surface of the substrate 4, at the position not occupied by the supporting posts 3, further comprising an electrostatic electrode layer 5;
the electrostatic electrode layer 5 and the bottom electrode layers 1-3 constitute a pair of electrode pairs, with a gap left therebetween except for the neutral layer 2 to provide a vibration space and prevent electrostatic adsorption; the electrode pair can apply a direct current bias voltage and an alternating current coupling signal, and controls and excites the form of the composite film formed by the piezoelectric lamination 1 and the neutral layer 2 based on the electrostatic force effect.
In the invention, when the piezoelectric stack 1 and the neutral layer 2 form the composite film, if the thickness is larger, the difficulty of the driving of the electrostatic force below the composite film is increased, so that the upper electrode layer 1-1 and the piezoelectric layer 1-2 above the piezoelectric stack 1 are further removed on the basis of the electrostatic electrode layer 5, and the device can work under the driving mechanism of the electrostatic force with higher performance.
In the present invention, in order to optimize the excitation efficiency of the structure, the upper electrode layer 1-1 is patterned to partially cover the piezoelectric layer 1-2, so that when an electrical signal is applied, only the local position of the piezoelectric layer 1-2 covered by the upper electrode layer 1-1 is subjected to the change of the electric field intensity, thereby responding to the locally enhanced strain; when the transducer array is formed, the upper electrode layers 1-1 included in each basic cell are all connected or separated; furthermore, the piezoelectric layer 1-2 is partially etched according to the pattern of the upper electrode layer 1-1, so that the overall structure of the array has the characteristic of unevenness, such as a thicker film at an effective vibration position and a thinner film at a structural support position, the rigidity characteristic of the overall structure is optimized, and the vibration sensitivity is further improved.
In the invention, for the linear array transducer or the two-dimensional array transducer formed by linear array continuation, the basic repeating units are strip-shaped array elements which are arranged in the vertical horizontal direction, the array elements and the fully-parallel transducer have the same characteristics, and the array elements are isolated in the following ways:
(1) the neutral layer 2, the bottom electrode layer 1-3 and the piezoelectric layer 1-2 are integrated, namely the components among the array elements are communicated with one another to form a flat surface, the basic units of the upper electrode layer 1-1 are mutually continuous in the array elements in electrical logic, or are connected in parallel through wiring layers during packaging after being separately arranged, the array elements are mutually separated, and the support column 3 only comprises a columnar microstructure;
(2) the neutral layer 2, the bottom electrode layer 1-3 and the piezoelectric layer 1-2 are integrated, namely, the components among the array elements are communicated with one another to form a flat surface, in electrical logic, basic units of the upper electrode layer 1-1 are mutually continuous in the array elements, or are connected in parallel through wiring layers during packaging after being arranged separately, the array elements are mutually separated, the support columns 3 are columnar microstructures in the array elements, and side walls are arranged among the array elements to provide support and physical boundary definition for a composite film of the array elements;
(3) the neutral layer 2, the bottom electrode layer 1-3 and the piezoelectric layer 1-2 are separated among the array elements, the piezoelectric layer 1-2 material, the bottom electrode layer 1-3 material and the neutral layer 2 material among the array elements are removed through an etching process, the substrate 4 below is exposed to form a high-isolation array, and all the parts in the array elements are communicated and are consistent with the structure of a full-parallel transducer.
In the invention, the thickness of the neutral layer 3 is 0.1-5 microns, the thickness of the piezoelectric layer 1-2 is 0.1-5 microns, the thickness of each electrode layer is 0.01-1 micron, the distance between the vertex angles of a convex polygon formed by the side boundaries of the basic units is 1-500 microns, the columnar micro mechanism contained in the support column 3 is a convex polygon column, the distance between the vertex angles is 1-100 microns, and the height is 0.05-100 microns; finally, the micromechanical ultrasonic transducer with the center frequency of 0.1-100 MHz is formed.
Further, the micromechanical transducer in the present invention further comprises an insulating layer, a wiring layer, a passivation layer and an acoustic matching layer formed above the piezoelectric stack 1 and the neutral layer 2; the wiring layer is made of metal materials, and the passivation layer, the insulating layer and the acoustic matching layer are made of insulating media. The insulating layer is used for isolating the wiring layer from the electrode layer; the wiring layer is used for forming connection with the electrode layer and leading out to an external pin so as to apply signals; the passivation layer is used for protecting the piezoelectric material and the metal material, so that the piezoelectric material and the metal material are isolated from external water vapor or other media, and the isolation and passivation effects are realized; the acoustic matching layer is used for improving the acoustic emission efficiency and is between the main body structure of the micromechanical ultrasonic transducer unit and a load medium (such as water, oil, air and the like).
When the micro-mechanical ultrasonic transducer based on the supporting column vibrates, the transducer array presents a cross-unit joint vibration mode, and the resonance mode is determined by the position characteristics of the supporting column of the point array. Conventional micromachined ultrasonic transducers have a support structure that is hollow, closed, circular or square in its interior, and each cell operates independently when vibrated. Compared with the prior art, the micromechanical ultrasonic transducer has the advantages that the area ratio of the vibration film is higher, higher sound pressure amplitude can be output when an array is formed to work, and higher peak value sensitivity is realized. Based on the support column array, heterogeneous packaging of the transducer and the CMOS chip can be further realized, so that channel uniformity is improved, energy dissipation caused by impedance mismatching in an electricity transmission process is reduced, and high-performance ultrasonic receiving and transmitting are realized.
Drawings
Figure 1 is a schematic diagram of a support post-based micromachined ultrasonic transducer array.
Fig. 2 is a schematic diagram of a basic unit structure of a support column-based micromachined ultrasonic transducer.
Fig. 3 is a schematic diagram of a support post-based micromachined ultrasonic transducer structure having an electrostatic electrode layer.
Fig. 4 is a schematic diagram of a support post scheme for a support post-based micromachined ultrasonic transducer.
Fig. 5 is a schematic diagram of a top electrode scheme for a support post based micromachined ultrasonic transducer.
Fig. 6 is a schematic diagram of an array scheme of a support post-based micromachined ultrasonic transducer.
FIG. 7 is a schematic diagram of the implementation of method 1.
FIG. 8 is a schematic diagram of the implementation of method 2.
FIG. 9 is a schematic diagram of the resonance mode of the array with regular triangle-shaped basic units according to the present invention.
Fig. 10 is a schematic diagram of resonant modes of an array with regular triangle boundaries in a conventional micromachined ultrasonic transducer.
FIG. 11 is a schematic diagram of the resonance mode of the array with square basic cells according to the present invention. .
Fig. 12 is a schematic diagram of the resonance mode of the array in which the basic unit is a regular hexagon. .
Reference numbers in the figures: 1 is a piezoelectric stack, 1-1 is an upper electrode layer, 1-2 is a piezoelectric layer, and 1-3 is a bottom electrode layer; 2 is a neutral layer; 3 is a support pillar, 4 is a substrate, and 5 is an electrostatic electrode layer.
Detailed Description
The invention will be described in more detail below with reference to the accompanying drawings. Like elements are represented by like numbers in the various figures. For purposes of clarity, the various features in the drawings are not necessarily drawn to scale. Moreover, some well-known elements may not be present.
Figure 1 is a schematic diagram of a support post-based micromachined ultrasonic transducer array. The piezoelectric ceramic comprises a piezoelectric stack 1, a neutral layer 2, support pillars 3 and a substrate 4; the piezoelectric stack comprises a thin upper electrode layer 1-1, a piezoelectric layer 1-2 and a bottom electrode layer 1-3 from top to bottom; the neutral layer 2 is arranged below the bottom electrode layers 1-3 and provides a supporting function for the piezoelectric stack 1; the supporting columns 3 are positioned below the neutral layer 2, the plurality of columnar structures are distributed into convex polygons, and the centers of the convex polygons are positioned at the vertex angles of the convex polygons to support the neutral layer; the substrate 4 is positioned below the support column 3 and provides a fixed support for the whole structure;
referring to an array schematic diagram 1 and a basic unit schematic diagram 2, the support pillar based micro-mechanical ultrasonic transducer provided by the invention is a planar array formed by splicing and extending basic units, and comprises: the array formed by splicing the basic units in full parallel, or a linear array element formed by splicing a plurality of basic units in parallel, and then extending the formed linear array, or a two-dimensional array formed by extending the full parallel array; the basic unit has a structure shown in fig. 2 and comprises a piezoelectric stack, a neutral layer, a support pillar and a substrate. The piezoelectric stack comprises a thin upper electrode layer, a piezoelectric layer and a bottom electrode layer from top to bottom; the neutral layer is arranged below the bottom electrode layer and provides a supporting function for the piezoelectric lamination; the supporting columns are positioned below the neutral layer, the plurality of columnar structures are distributed into convex polygons, and the centers of the convex polygons are positioned at the vertex angles of the convex polygons to provide support for the neutral layer; the substrate is positioned below the support columns and provides a fixing and supporting effect for the whole structure;
the lateral boundary range of the basic units of the transducer is a convex polygon and is marked by the centers of a plurality of supporting columns, when a linear array or a full parallel transducer is formed, polygonal basic units such as triangles and quadrangles are combined, finally, the array of the supporting columns is formed in the horizontal direction, an integrated or separated neutral layer and a piezoelectric lamination layer are supported above the array, and an integrated substrate is combined below the array to form the linear array or the full parallel transducer; from another perspective, the most important feature of the transducer array of the present invention is that the array of support pillars has a certain arrangement, so as to implement a cross-cell resonant mode with a certain center frequency.
The neutral layer provides support for each component of the piezoelectric lamination, so that when the piezoelectric lamination is excited to generate strain, a strain neutral surface of a composite film formed by the piezoelectric lamination and the neutral layer is positioned in the neutral layer; and a flat contact surface is formed between the neutral layer and the support column, and the neutral layer and the support column are combined through a bonding process.
The specific geometric design parameters of the micromechanical ultrasonic transducer provided by the invention are as follows: the thickness of the neutral layer 3 is 0.1-5 microns, the thickness of the piezoelectric layer 1-2 is 0.1-5 microns, the thickness of each electrode layer is 0.01-1 micron, the distance between the vertex angles of the convex polygons formed by the side boundaries of the basic units is 1-500 microns, the columnar micro mechanism contained in the support column 3 is a convex polygon column, the distance between the vertex angles is 1-100 microns, and the height is 0.05-100 microns; the transducer with the center frequency of 0.1-100 MHz is finally formed.
Referring to the structural schematic diagram of the support pillar based micromachined ultrasonic transducer with the electrostatic electrode layer shown in fig. 3, the present invention further covers the electrostatic electrode layer 5 on the surface of the substrate 4 and the unoccupied position of the support pillar 3 on the basis of the micromachined ultrasonic transducer shown in fig. 1 and 2, so as to introduce the electrostatic force effect, and the sensitivity of the micromachined ultrasonic transducer can be further enhanced by working together with the piezoelectric effect. The electrostatic electrode layer 5 and the bottom electrode layers 1-3 form a pair of electrode pairs, and a gap is reserved between the electrode pairs to provide a vibration space and prevent electrostatic adsorption; the electrode pair can apply a direct current bias voltage and an alternating current coupling signal, and controls and excites the form of the composite film formed by the piezoelectric lamination 1 and the neutral layer 2 based on the electrostatic force effect.
Referring to the schematic diagram of the supporting column scheme shown in fig. 4, the supporting columns 3 are arranged as follows: when a parallel structure of array elements in a fully parallel transducer or a linear array transducer is formed, the supporting columns 3 comprise cylindrical microstructure arrays with the same height, and when the supporting capability needs to be enhanced, side walls outside the outermost side boundaries of the arrays are further included. As shown in fig. 4a, the array of cylindrical microstructures is regularly arranged on the substrate 4, and the position of each unit is adjustable, preferably, the array includes the same regular triangle mosaic arrangement, the same square mosaic arrangement, and the same regular hexagon mosaic arrangement. When the cylindrical microstructure array is spliced and filled with basic units with different sizes and convex polygonal shapes, a transducer array with broadband or local resonance reinforcement can be formed; as shown in fig. 4b, the inner boundary of the sidewall is a closed figure, which encloses a plurality of transducer elements, and the outer portion of the sidewall continuously extends to another array, or a cut-out of the transducer chip. Because the side wall and the neutral plane have larger contact area, the side wall can provide higher-strength support for the vibration film so as to improve the reliability of the long-time work of the transducer.
Referring to the schematic top electrode scheme shown in fig. 5, the top electrode is designed in the following manner: the local part of the array is provided with an integrated bottom electrode layer 1-3 and piezoelectric layer 1-2, when the upper electrode layer 1-1, the bottom electrode layer 1-3 and the piezoelectric layer 1-2 are close to each other, the electrode pair formed by the upper electrode layer 1-1 and the bottom electrode layer 1-3 applies a continuous electrostatic field and an alternating electric field with uniform amplitude on the piezoelectric layer 1-2; in order to locally enhance the electric field at the membrane vibration, remove the electric field in the inert areas or locations where the in-phase piezoelectric layer is not required to be strained, the top electrode layer 1-1 can be etched for further patterning, as shown in fig. 5a and 5 b; in fig. 5a, the center of the free portion of the composite film and the cross-cell communication portion cover the upper electrode layer 1-1, the electrode material is removed above the support posts 3, and the upper electrode layer 1-1 is integrally communicated to form a fully parallel array; in fig. 5b, only the center of the free portion of the composite film is covered with the upper electrode layer 1-1, the upper electrode layer 1-1 is entirely discrete, and the units are reconfigured by vias and routing layers in the subsequent packaging process to form the desired line array or fully parallel sheets. It should be noted that the configuration of the top electrode is not limited to the above two manners, and only a schematic diagram is provided to show a possible patterning route.
Referring to the schematic diagram of the array scheme shown in fig. 6, the array topology scheme that can be implemented by the micromachined ultrasonic transducer according to the present invention is as follows: when the linear array transducer is formed, the basic repeating units of the array are strip-shaped array elements which are arranged in the vertical horizontal direction, the characteristics of the array elements are consistent with those of the fully-parallel transducer, and the array elements are isolated in the following modes:
(1) the neutral layer 2, the bottom electrode layer 1-3 and the piezoelectric layer 1-2 are integrated, namely the components among the array elements are communicated with one another to form a flat surface, the basic units of the upper electrode layer 1-1 are mutually continuous in the array elements in electrical logic, or are connected in parallel through wiring layers during packaging after being arranged separately, the basic units are mutually separated among the array elements, and the supporting column 3 only comprises a columnar microstructure;
(2) the neutral layer 2, the bottom electrode layer 1-3 and the piezoelectric layer 1-2 are integrated, namely, the components among the array elements are communicated with one another to form a flat surface, in electrical logic, basic units of the upper electrode layer 1-1 are mutually continuous in the array elements, or are connected in parallel through wiring layers during packaging after being arranged separately, the array elements are mutually separated, the support columns 3 are columnar microstructures in the array elements, and side walls are arranged among the array elements to provide support and physical boundary definition for a composite film of the array elements;
(3) the neutral layer 2, the bottom electrode layer 1-3 and the piezoelectric layer 1-2 are separated among the array elements, the piezoelectric layer 1-2 material, the bottom electrode layer 1-3 material and the neutral layer 2 material among the array elements are removed through an etching process, the substrate 4 below is exposed to form a high-isolation array, and all the parts in the array elements are communicated and are consistent with the structure of a full-parallel transducer.
Referring to implementation method 1 shown in fig. 7, an embodiment of the micromachined ultrasonic transducer according to the present invention is specifically described as follows: an alternating current signal source is loaded on an electrode pair formed by an upper electrode layer 1-1 and a bottom electrode layer 1-3 of the piezoelectric lamination 1, the piezoelectric layer has strain response along with the change of an electric field, and the neutral layer is driven to be extruded or expanded through the stretching or the contraction of the piezoelectric layer, so that the structure presents driven vibration of the semi-fixed thin plate. When the outside is loaded with sound-conducting medium, such as water and oil, the membrane vibrates to press the sound-conducting medium, so that the emission of ultrasonic waves is realized, and vice versa.
Referring to the embodiment 2 shown in fig. 8, a dc bias voltage is applied between the electrode pair formed between the electrostatic electrode layer 5 and the bottom electrode layers 1-3, the bottom electrode layers 1-3 are defined as ground, the dc bias voltage is typically tens of volts to hundreds of volts, which is used to form charge accumulation between the electrode pairs as a basis for ac signal excitation, and the electrostatic force formed between the electrode plates is used to pull the composite film to the bottom of the capacitor cavity, thereby increasing the elasticity of the film and adjusting the resonant frequency;
secondly, an alternating current signal source is applied between the electrode pair formed by the electrode layer 1-1 and the bottom electrode layer 1-3, electric field change is generated between the electrodes through instantaneous voltage change, the piezoelectric material generates strain response based on the inverse piezoelectric effect of the piezoelectric material and drives the composite film to bend, in addition, the electric charge change generated on the bottom electrode layer 1-3 influences the fluctuation of electrostatic force and drives the composite film to bend, the two effects jointly drive the film, and when the instantaneous structure bending change caused by the inverse piezoelectric effect is consistent with the instantaneous structure bending change direction caused by the electrostatic force, the structural amplitude is greatly improved, and the high-sensitivity micro-mechanical ultrasonic transducer is formed. When the ultrasonic transducer is used for ultrasonic signal receiving, an electrode pair formed by the electrode layers 1-1 and 1-3 is disconnected from an alternating current signal source through a switch circuit and is further connected with a receiving front-end circuit, and the receiving front-end circuit is a voltage amplifier or a transconductance amplifier which is formed by a low-noise amplifier.
Referring to the amplitude curve and the vibration mode simulation result of the supporting pillar-based equilateral triangle micro-machined ultrasonic transducer array shown in fig. 9, and comparing with the amplitude curve and the vibration mode simulation result of the conventional fully-fixed equilateral triangle micro-machined ultrasonic transducer array shown in fig. 10; for a square micromachined ultrasonic transducer array based on support posts, the simulation results are shown in fig. 11; for a support pillar based regular hexagonal micromachined ultrasonic transducer array, the simulation results are shown in fig. 12; the specific results are as follows:
in a simulation model, the lengths from the center to each vertex angle of a designed regular triangle, square and regular hexagon micro-mechanical ultrasonic transducer are 50 micrometers, the thickness of a piezoelectric layer is 1 micrometer, the thickness of a neutral layer is 3 micrometers, the thickness of an electrode layer is not counted, for a model based on a support column, the radius of the model is 5 micrometers, and the model is applied to a circular surface at the vertex angle of a bottom boundary of the model under the condition of fixing the boundary; for a fully-fixed model, fixed boundary conditions are imposed on the torus within 5 microns of the bottom boundary of the model. Applying an axisymmetric periodic boundary condition to the side boundary of the convex polygon basic unit;
in simulation, alternating current signals with unit amplitude are applied to the piezoelectric layer, frequency domain analysis is carried out on a model at 0-40 MHz, average displacement frequency response curves of the surface of the membrane are respectively shown in FIGS. 9-12, wherein the triangular micro-mechanical ultrasonic transducer based on the supporting column has 4 resonance peaks at 0-40 MHz, a basic resonance mode is located at 4.72MHz, the position of the maximum displacement is the center of a basic unit at the center frequency, meanwhile, the position of a cross-unit, namely, the supporting column has large displacement, and the whole membrane has large vibration area; the second resonance mode is at 26.05MHz, the center frequency, and the array exhibits a ring disk vibration across the cell; the third and fourth resonant modes are positioned at 39.64MHz and 39.93 MHz; high-order resonance which is mainly in the cell boundary is adopted, and the sensitivity is low;
correspondingly, the resonance mode of the fully-fixed micro-mechanical ultrasonic transducer is single, only 1 resonance front exists between 0MHz and 40MHz, and the resonance mode is located at 24.54MHz, so that although the unit has the same area as the transducer provided by the invention, the resonance frequency is too high, the size requirement of a phased array is not favorably met, the effective area of vibration is small, the filling efficiency is low, and the output of high sound pressure of the traditional transducer is influenced;
similarly, square, regular hexagonal, post-based micromachined ultrasonic transducers, see fig. 11 and 12, all have 2 to 3 characteristic resonant modes in the range of 0-40 MHz, and similar to triangular post-based micromachined ultrasonic transducers, the fundamental resonant mode is located at 6-7 MHz, exhibiting cross-cell, high effective area resonance, with higher order frequencies also having cross-cell characteristics.
Claims (10)
1. A micromechanical ultrasonic transducer based on supporting columns is characterized in that a planar array formed by splicing and extending basic units comprises: the array formed by splicing the basic units in full parallel, or a linear array element formed by splicing a plurality of basic units in parallel, and then extending the formed linear array, or a two-dimensional array formed by extending the full parallel array; the structure of the basic unit comprises a piezoelectric stack (1), a neutral layer (2), a support pillar (3) and a substrate (4); the piezoelectric stack comprises a thin upper electrode layer (1-1), a piezoelectric layer (1-2) and a bottom electrode layer (1-3) from top to bottom; the neutral layer (2) is arranged below the bottom electrode layer (1-3) and provides a supporting function for the piezoelectric lamination (1), and when the piezoelectric lamination (1) is excited to generate strain, a strain neutral plane of a composite film formed by the piezoelectric lamination (1) and the neutral layer (2) is positioned in the neutral layer (2); the supporting column (3) is positioned below the neutral layer (2) and provides support for the neutral layer; the substrate (4) is positioned below the support columns (3) and provides fixed support for the whole structure;
an upper electrode layer (1-1) and a bottom electrode layer (1-3) of the piezoelectric lamination layer (1) form an electrode pair for applying and collecting an electric field, electric charges and voltage on the piezoelectric layer (1-2); wherein the piezoelectric stack (1) can be statically controlled by applying a dc bias voltage; dynamically exciting the piezoelectric stack (1) by applying an alternating current coupling signal;
the peripheral outline of the structure of the basic unit is a convex polygon.
2. The post-based micromachined ultrasonic transducer of claim 1, wherein the basic cell shape is regular triangle, square, regular hexagon.
3. The support pillar based micromachined ultrasonic transducer of claim 2, being a planar array structure formed by the assembly and splicing of basic cells of one shape; or a plane array structure formed by combining and splicing basic units with different shapes; the side lengths of the basic units in different shapes are matched with each other, so that the boundaries among the different units are spliced tightly.
4. The post-based micromachined ultrasonic transducer of claim 3, which is an array of lines formed by extended arrangement of single-shaped base units; or the basic units with different shapes are combined and spliced to form a linear array, wherein the side lengths are mutually matched, so that the boundaries among the different units are tightly spliced; or the linear arrays are subjected to periodic continuation to form a two-dimensional array structure.
5. Support pillar based micromachined ultrasonic transducer according to one of claims 1 to 4, wherein the support pillar (3) is a discrete columnar structure, i.e. within one elementary cell, between the substrate (4) and the neutral layer (2); or the supporting columns (3) are discrete columnar structures inside the basic units, and are strip-shaped frame structures, also called side walls, at the boundaries of the array formed by the combination of the basic units.
6. Support post based micromachined ultrasonic transducer according to claim 5, wherein the substrate (4) is used as a structure having supporting and electrical connecting functions, either as a bare wafer without circuit function or as a wafer with circuit function;
when the substrate (4) is a bare wafer without circuit function, the upper support pillar (3) is formed by etching the bare wafer through a top-down patterning process and forms an integrated structure with the substrate (4), or the original photoresist pattern is etched or stripped through a bottom-up deposition process to form a columnar microstructure above the substrate (4).
7. Support post based micromachined ultrasonic transducer according to one of claims 1 to 4 or 6, wherein at the surface of the substrate (4), at the unoccupied position of the support post (3), an electrostatic electrode layer (5) is provided, the electrostatic electrode layer (5) and the bottom electrode layer (1-3) constituting a pair of electrode pairs; the sensitivity of the micromechanical ultrasonic transducer is further enhanced by introducing an electrostatic force effect, working together with a piezoelectric effect.
8. The post based micromachined ultrasonic transducer of claim 7, wherein for a two dimensional array transducer formed by a line array transducer and a line array continuation, there are several isolation ways between array elements:
(1) the neutral layer (2), the bottom electrode layer (1-3) and the piezoelectric layer (1-2) are integrated, namely the components among the array elements are communicated with one another to form a flat surface, in electrical logic, basic units of the upper electrode layer (1-1) are mutually continuous in the array elements or are connected in parallel through wiring layers during packaging after being arranged in a discrete mode, the array elements are mutually discrete, and the supporting column (3) only comprises a columnar microstructure;
(2) the neutral layer (2), the bottom electrode layer (1-3) and the piezoelectric layer (1-2) are integrated, namely the components among the array elements are communicated with one another to form a flat surface, in electrical logic, basic units of the upper electrode layer (1-1) are mutually continuous in the array elements, or are connected in parallel through wiring layers during packaging after being arranged separately, the array elements are mutually separated, the support columns (3) are columnar microstructures in the array elements, and side walls are arranged among the array elements to provide support and physical boundary definition for a composite film of the array elements;
(3) the neutral layer (2), the bottom electrode layer (1-3) and the piezoelectric layer (1-2) are separated among the array elements, the piezoelectric layer (1-2) material, the bottom electrode layer (1-3) material and the neutral layer (2) material among the array elements are removed through an etching process, the lower substrate (4) is exposed to form a high-isolation array, and all the parts are communicated in the array elements.
9. Support post based micromachined ultrasonic transducer according to one of claims 1 to 4 or 6, wherein the thickness of the neutral layer (3) is 0.1 to 5 micrometers, the thickness of the piezoelectric layers (1 to 2) is 0.1 to 5 micrometers, and the thickness of each electrode layer is 0.01 to 1 micrometer.
10. A post-support based micromachined ultrasonic transducer according to any one of claims 1 to 4 or 6, further comprising an insulating layer, a wiring layer, a passivation layer and an acoustic matching layer formed over the piezoelectric stack (1) and the neutral layer (2); the wiring layer is made of metal materials, and the passivation layer, the insulating layer and the acoustic matching layer are insulating media; the insulating layer is used for isolating the wiring layer from the electrode layer; the wiring layer is used for forming connection with the electrode layer and leading out to an external pin so as to apply signals; the passivation layer is used for protecting the piezoelectric material and the metal material, so that the piezoelectric material and the metal material are isolated from external water vapor or other media, and isolation and passivation are realized; the acoustic matching layer is arranged between the main body structure of the micro-mechanical ultrasonic transducer unit and the load medium and used for improving the acoustic emission efficiency.
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