CN115532572B - Multi-frequency piezoelectric micromechanical ultrasonic transducer and preparation method thereof - Google Patents
Multi-frequency piezoelectric micromechanical ultrasonic transducer and preparation method thereof Download PDFInfo
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B06—GENERATING OR TRANSMITTING MECHANICAL VIBRATIONS IN GENERAL
- B06B—METHODS OR APPARATUS FOR GENERATING OR TRANSMITTING MECHANICAL VIBRATIONS OF INFRASONIC, SONIC, OR ULTRASONIC FREQUENCY, e.g. FOR PERFORMING MECHANICAL WORK IN GENERAL
- 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
- B06B1/0644—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 using a single piezoelectric element
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B81—MICROSTRUCTURAL TECHNOLOGY
- B81C—PROCESSES OR APPARATUS SPECIALLY ADAPTED FOR THE MANUFACTURE OR TREATMENT OF MICROSTRUCTURAL DEVICES OR SYSTEMS
- B81C1/00—Manufacture or treatment of devices or systems in or on a substrate
- B81C1/00015—Manufacture or treatment of devices or systems in or on a substrate for manufacturing microsystems
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Abstract
The invention relates to a multi-frequency piezoelectric micromechanical ultrasonic transducer and a preparation method thereof, which relate to the field of transducers, wherein the transducer comprises: the piezoelectric device comprises a top electrode layer, a piezoelectric layer, a bottom electrode layer, an insulating layer, a structural layer and a matrix which are sequentially arranged; two Helmholtz resonant cavities are arranged on the substrate; the two Helmholtz resonant cavities are communicated through the neck channel; the structural layer covers the neck passage and both of the helmholtz resonators. The invention combines the Helmholtz resonant cavity with the piezoelectric film, and can realize a plurality of resonant frequencies of the transducer.
Description
Technical Field
The invention relates to the field of transducers, in particular to a multi-frequency piezoelectric micromechanical ultrasonic transducer and a preparation method thereof.
Background
The traditional ultrasonic transducers using the bulk piezoelectric ceramics as materials only work at the first-order resonance, which causes the ultrasonic transducers to realize the switching of the working frequency by switching probes or manufacturing the ultrasonic transducers into a combined structure containing a plurality of single-product transducers, thereby greatly increasing the complexity of production and manufacturing and reducing the effect of beam synthesis. Meanwhile, the traditional massive piezoelectric ceramic material has the defects of poor matching degree with tissue acoustic impedance and expensive array processing. In recent years, with the development of microelectromechanical systems (MEMS) technology, a micromechanical transducer (MUTs) is capable of exciting multiple operating frequencies on one vibrating diaphragm, while the compliant membrane structure of MUTs has a low acoustic impedance, enables good coupling with a medium, and facilitates manufacturing of a large array of compact design. According to the working principle, MUTs is generally divided into two types, namely a Capacitive Micromachined Ultrasonic Transducer (CMUT) and a Piezoelectric Micromachined Ultrasonic Transducer (PMUT). In contrast to CMUTs, PMUTs do not require high voltage bias sources nor leave capacitive gaps. At present, piezoelectric micro-mechanical transducers based on aluminum nitride films are the leading edge of research, and some PMUT devices with multiple working frequencies exist at present, but most focus on the aspects of mechanical domain and electrical domain, such as changing the shape of a vibrating diaphragm and optimizing electrode configuration, and few consideration is given to realizing multi-frequency vibration in the acoustic domain by utilizing the coupling effect between the diaphragm and a medium.
Disclosure of Invention
The invention aims to provide a multi-frequency piezoelectric micromechanical ultrasonic transducer and a preparation method thereof, wherein a Helmholtz resonant cavity is combined with a piezoelectric film, so that a plurality of resonant frequencies of the transducer are realized.
In order to achieve the above object, the present invention provides the following solutions:
a multi-frequency piezoelectric micromachined ultrasonic transducer comprising: the piezoelectric device comprises a top electrode layer, a piezoelectric layer, a bottom electrode layer, an insulating layer, a structural layer and a matrix which are sequentially arranged;
two Helmholtz resonant cavities are arranged on the substrate; the two Helmholtz resonant cavities are communicated through the neck channel; the structural layer covers the neck passage and both of the helmholtz resonators.
Optionally, both the helmholtz resonators are cuboid cavities.
Optionally, the top electrode layer, the piezoelectric layer, the bottom electrode layer and the insulating layer are provided with two groups; each group of top electrode layers, the piezoelectric layers, the bottom electrode layers and the insulating layers which are sequentially arranged correspond to one Helmholtz resonant cavity.
Optionally, the material of the piezoelectric layer is aluminum nitride.
Optionally, the material of the structural layer is silicon.
Optionally, the material of the insulating layer is silicon dioxide.
The invention also provides a preparation method of the multi-frequency piezoelectric micromechanical ultrasonic transducer, which is used for preparing the multi-frequency piezoelectric micromechanical ultrasonic transducer, and comprises the following steps:
Etching two Helmholtz resonant cavities on a substrate and etching a communicated neck channel between the two Helmholtz resonant cavities;
Preparing a structural layer on the etched substrate by using a vapor deposition method;
Sequentially growing a bottom electrode layer, a piezoelectric layer and a top electrode layer on the structural layer by utilizing a magnetron sputtering process;
etching the piezoelectric layer and the top electrode layer by using a plasma etching method;
And depositing an insulating layer on the surface of the bottom electrode layer to obtain the multi-frequency piezoelectric micromechanical ultrasonic transducer.
Optionally, the preparing the structural layer for the etched substrate by using a vapor deposition method specifically includes:
utilizing a wet silicon oxide layer for the etched substrate;
Preparing polycrystalline silicon on the oxidized silicon layer by using a vapor deposition method as a sacrificial layer and polishing the surface of the sacrificial layer;
And preparing a structural layer on the sacrificial layer after surface polishing by using a vapor deposition method and carrying out surface polishing on the structural layer.
Optionally, after the bottom electrode layer, the piezoelectric layer and the top electrode layer are sequentially grown on the structural layer by using a magnetron sputtering process, the method further comprises:
etching the through hole to the sacrificial layer by using a dry method;
etching the sacrificial layer to the piezoelectric layer by utilizing xenon difluoride solution to obtain a back cavity structure combined with the neck channel;
And filling the through holes.
According to the specific embodiment provided by the invention, the invention discloses the following technical effects:
The invention comprises the following steps: the piezoelectric device comprises a top electrode layer, a piezoelectric layer, a bottom electrode layer, an insulating layer, a structural layer and a matrix which are sequentially arranged; two Helmholtz resonant cavities are arranged on the substrate; the two Helmholtz resonant cavities are communicated through the neck channel; the structural layer covers the neck passage and both of the helmholtz resonators. The invention combines the Helmholtz resonant cavity with the piezoelectric film to realize a plurality of resonant frequencies of the transducer.
Drawings
In order to more clearly illustrate the embodiments of the present invention or the technical solutions of the prior art, the drawings that are needed in the embodiments will be briefly described below, it being obvious that the drawings in the following description are only some embodiments of the present invention, and that other drawings may be obtained according to these drawings without inventive effort for a person skilled in the art.
FIG. 1 is a schematic diagram of a multi-frequency piezoelectric micromechanical ultrasonic transducer;
Fig. 2 is a block diagram of two conventional adjacent square PMUTs;
FIG. 3 is a block diagram of two adjacent PMUT cavities connected together by a neck channel;
FIG. 4 is a schematic diagram of an equivalent circuit;
FIG. 5 is a diagram showing resonance and emission sound pressure at different sizes;
Fig. 6 is a schematic diagram of the emission bandwidth of an in-air device.
Symbol description:
The device comprises a 1-top electrode layer, a 2-piezoelectric layer, a 3-bottom electrode layer, a 4-insulating layer, a 5-structural layer, a 6-substrate, a 7-Helmholtz resonant cavity and an 8-square PMUT.
Detailed Description
The following description of the embodiments of the present invention will be made clearly and completely with reference to the accompanying drawings, in which it is apparent that the embodiments described are only some embodiments of the present invention, but not all embodiments. All other embodiments, which can be made by those skilled in the art based on the embodiments of the invention without making any inventive effort, are intended to be within the scope of the invention.
The invention aims to provide a multi-frequency piezoelectric micromechanical ultrasonic transducer and a preparation method thereof, and provides a method for combining a Helmholtz resonant cavity with a piezoelectric film to realize a plurality of resonant frequencies of the transducer.
In order that the above-recited objects, features and advantages of the present invention will become more readily apparent, a more particular description of the invention will be rendered by reference to the appended drawings and appended detailed description.
As shown in fig. 1 to 3, the present invention provides a multi-frequency piezoelectric micromachined ultrasonic transducer, including: a top electrode layer 1, a piezoelectric layer 2, a bottom electrode layer 3, an insulating layer 4, a structural layer 5 and a substrate 6 which are sequentially arranged.
Two Helmholtz resonant cavities 7 are arranged on the substrate 6; the two Helmholtz resonant cavities 7 are communicated through a neck passage; the structural layer 5 covers the neck passage and the two helmholtz resonators 7.
Wherein, two helmholtz resonant cavities 7 are cuboid cavities. The top electrode layer 1, the piezoelectric layer 2, the bottom electrode layer 3 and the insulating layer 4 are provided with two groups; each group of top electrode layers 1, piezoelectric layers 2, bottom electrode layers 3 and insulating layers 4 which are sequentially arranged corresponds to one Helmholtz resonant cavity 7. The material of the piezoelectric layer 2 is aluminum nitride. The material of the structural layer 5 is silicon. The material of the insulating layer 4 is silicon dioxide.
The multi-frequency piezoelectric micromechanical ultrasonic transducer provided by the invention is characterized in that back cavities of two adjacent PMUT units are connected together through a narrow channel, so that a cavity-neck-cavity series type Helmholtz resonant cavity is formed, and the cavity is shown in figure 1.
The entire device comprises a conventional square PMUT8 of piezoelectric thin film sandwich sandwiched between top and bottom electrodes on a structural layer 5. The electrode layer is used for receiving external excitation voltage; the piezoelectric layer 2 is made of piezoelectric material aluminum nitride and is used for converting electromechanical energy; the structural layer 5 silicon acts as a support layer, increasing the stability of the device, while the insulating layer 4 uses silicon dioxide as insulating material between the bottom electrode layer 3 and the structural layer 5. When a voltage is applied to the electrode layer, the bending moment generated by the d31 piezoelectric effect deflects the diaphragm, so that the vibration of air in the series Helmholtz resonant cavities 7 is driven, and the pressure in the cavities can be reacted to the diaphragm, so that the influence on the emission sound pressure is generated. A 0 in fig. 2 is the top electrode diameter; h 1 is the piezoelectric layer thickness; h 2 is the structural layer thickness, a 1 in fig. 3 is the cavity diameter; t is the cavity height; t n is the height of the neck cavity; w n is the width of the neck cavity.
The equivalent circuit model of the multi-frequency PMUT device is shown in fig. 4, where the acoustic resistance is increased due to the introduction of the resonant cavity. U in fig. 4 is the velocity of the mechanical domain and U is the velocity of the acoustic domain. Wherein the PMUT cell is driven by a voltage Vin, C0 is a feedthrough capacitance, and the electromechanical coupling is defined by a rotation ratio η. Mm, cm, rm are the mass, capacitance and mechanical damping of the PMUT cell, respectively. Coupling occurs through the surface area Aeff from the mechanical domain to the acoustic domain. The free radiation impedance Z free of the square clamped PMUT in the acoustic domain is: Wherein ρ 0,c0 is the dielectric density and the sound velocity, R free,Xfree is the free radiation resistance and the free radiation reactance, and j is the imaginary number.
The acoustic impedance Z ha of the series-connected helmholtz resonator 7 is: Wherein ω is angular frequency, M ha is acoustic mass, C ha,1 is acoustic capacitance of the left cavity, C ha,2 is acoustic capacitance of the right cavity, and the cavity has acoustic capacitance/>, as a capacitor The neck is denoted by acoustic mass M ha as an inductor,/>L n is the length of the neck cavity, while the Helmholtz cavity has an acoustic resistanceWherein V 1,V2 is the volume of the left and right cavity, S 1 is the cross-sectional area of the cavity, and k is the wave number.
The mutual radiation impedance Zpp between two adjacent PMUT array elements is expressed as: d is the spacing between two adjacent PMUT array elements.
The device can be prepared by MEMS manufacturing process to realize improvement of multiple working frequencies and emission performance.
The invention also provides a preparation method of the multi-frequency piezoelectric micromechanical ultrasonic transducer, which is used for preparing the multi-frequency piezoelectric micromechanical ultrasonic transducer, and comprises the following steps:
Two Helmholtz resonators are etched on the substrate and a neck passage is etched in communication between the two Helmholtz resonators.
And preparing a structural layer on the etched substrate by using a vapor deposition method.
And sequentially growing a bottom electrode layer, a piezoelectric layer and a top electrode layer on the structural layer by utilizing a magnetron sputtering process.
And etching the piezoelectric layer and the top electrode layer by using a plasma etching method.
And depositing an insulating layer on the surface of the bottom electrode layer to obtain the multi-frequency piezoelectric micromechanical ultrasonic transducer.
In practical application, the method for preparing the structural layer for the etched substrate by using the vapor deposition method specifically comprises the following steps:
And utilizing a wet silicon oxide layer for the etched substrate.
Polysilicon is prepared as a sacrificial layer on the oxidized silicon layer by a vapor deposition method and the sacrificial layer is subjected to surface polishing.
And preparing a structural layer on the sacrificial layer after surface polishing by using a vapor deposition method and carrying out surface polishing on the structural layer.
In practical application, after the bottom electrode layer, the piezoelectric layer and the top electrode layer are sequentially grown on the structural layer by using the magnetron sputtering process, the method further comprises:
And etching the through hole to the sacrificial layer by using a dry method.
And etching the sacrificial layer to the piezoelectric layer by utilizing xenon difluoride solution to obtain the back cavity structure combined with the neck channel.
And filling the through holes.
The invention also provides a specific working flow of the preparation method of the multi-frequency piezoelectric micromachined ultrasonic transducer in practical application, which comprises the following steps:
step 1: preparing an SOI substrate with the thickness of 3 mu m on a 500nm deep buried oxide as a base wafer material, and cleaning the SOI substrate by using a standard silicon wafer cleaning process;
Step 2: etching a Helmholtz cavity with the depth of 4 microns and the length of 180 microns on the surface of the wafer, and etching Helmholtz neck channels with the width of 50 microns and the length of 80 microns in two adjacent cavities;
step 3: using a wet thermal silicon oxide layer for the device of the step 2;
step 4: preparing polysilicon as a sacrificial layer by using a low-temperature plasma enhanced chemical vapor deposition method, and performing a surface polishing process;
Step 5: using 300nm silicon dioxide prepared by a plasma enhanced chemical vapor deposition method as a supporting layer, and polishing the surface;
Step 6: growing a molybdenum bottom electrode with the thickness of 200nm on the device in the step 5 through a magnetron sputtering process, performing magnetron sputtering on 1.8 mu m aluminum nitride (002 oriented crystal is used) on the bottom electrode to serve as a piezoelectric layer, and performing magnetron sputtering on 200nm molybdenum on the piezoelectric layer to serve as a top electrode layer;
Step 7: dry etching the through hole to the polycrystalline silicon sacrificial layer;
Step 8: etching the sacrificial layer by using xenon difluoride solution, thereby releasing the piezoelectric layer and forming a back cavity structure with a cavity combined with the Helmholtz neck;
step 9: filling and etching the through holes;
Step 10: patterning the top electrode, the piezoelectric layer and the ground via using a plasma etch: firstly, using a plasma vapor deposition method to deposit 500nm silicon dioxide as a hard mask, then carrying out wet etching on the silicon dioxide, and etching aluminum nitride by using chlorine-based plasma; then wet etching is performed in the developer solution MF-319 to pattern the ground via hole. The top electrode was designed with an average radius of 70% (relative to the released diaphragm) and the axial diaphragm coverage rate was 55%, as preliminary results using finite element analysis indicate that good coupling to the fundamental vibration mode of the diaphragm could be achieved in this case;
Step 11: and (3) depositing silicon nitride on the surface of the device in the step (10) as an insulating layer, leading out an upper electrode and carrying out high polymer coating.
The multi-frequency piezoelectric micromechanical ultrasonic transducer provided by the invention has the following advantages:
1. Easy to adjust the resonance frequency
The multi-frequency piezoelectric micromechanical ultrasonic transducer based on the Helmholtz resonator provided by the invention can obtain different resonance frequencies by changing the sizes of the Helmholtz cavity and the neck under the condition of not changing the size of the piezoelectric film. Fig. 5 is a view of the sound pressure of the transducer surface using finite element analysis software under conditions of varying the neck length, neck width, and cavity height of the helmholtz resonator. Fig. 5 (a) is a frequency response diagram of the surface average pressure when the neck length is changed from 30um to 70um, fig. 5 (b) is a frequency response diagram of the surface average pressure when the neck width is changed from 50um to 100um, and fig. 5 (c) is a frequency response diagram of the surface average pressure when the neck height is changed from 5um to 40um, it can be seen that the introduction of the helmholtz resonator generates two additional resonance peaks, and the resonance occurs at a position in relation to the sound pressure value at the resonance, which is in relation to the geometry. One preferred parameter of the structure of the invention is as follows: the length of the neck of the Helmholtz resonator is 50 μm, the width of the neck is 80 μm, and the height of the cavity is 4 μm.
2. Improved emission performance
To evaluate the transmission mode impulse response and performance of devices operating in air, transient analyses were performed on the devices of the present invention and compared to a conventional resonator-free PMUT of the same size. Both PMUTs are driven by 100V, 1.8MHz electrical pulse signals. The duration of the pulse signal is 120ns. The pulses are temporally varied by convolving with the same Blackman window. Conventional PMUTs have a sharp resonance peak curve with a bandwidth of only 5%, about 0.1MHz, and a center frequency of 1.82MHz. The structure of the invention forms ultra-wide frequency bands due to the combination of excitation modes. The maximum bandwidth of-6 dB can reach 77% (1.4 MHz), and the central frequency is 1.82MHz (77%).
The invention combines the Helmholtz resonant cavity and the piezoelectric film together, realizes a plurality of resonant frequencies, and obtains the increase of bandwidth and the great enhancement of emission sound pressure through the coupling effect.
In the present specification, each embodiment is described in a progressive manner, and each embodiment is mainly described in a different point from other embodiments, and identical and similar parts between the embodiments are all enough to refer to each other.
The principles and embodiments of the present invention have been described herein with reference to specific examples, the description of which is intended only to assist in understanding the core concept of the invention; also, it is within the scope of the present invention to be modified by those of ordinary skill in the art in light of the present teachings. In view of the foregoing, this description should not be construed as limiting the invention.
Claims (8)
1. A multi-frequency piezoelectric micromachined ultrasonic transducer comprising: the piezoelectric device comprises a top electrode layer, a piezoelectric layer, a bottom electrode layer, an insulating layer, a structural layer and a matrix which are sequentially arranged;
two Helmholtz resonant cavities are arranged on the substrate; the two Helmholtz resonant cavities are communicated through the neck channel; the structural layer covers the neck channel and the two helmholtz resonators;
The top electrode layer, the piezoelectric layer, the bottom electrode layer and the insulating layer are all provided with two groups; each group of the top electrode layer, the piezoelectric layer, the bottom electrode layer and the insulating layer which are sequentially arranged correspond to a Helmholtz resonant cavity.
2. The multi-frequency piezoelectric micromachined ultrasonic transducer of claim 1, wherein both of the helmholtz resonators are rectangular cavities.
3. The multi-frequency piezoelectric micromachined ultrasonic transducer of claim 1, wherein the material of the piezoelectric layer is aluminum nitride.
4. The multi-frequency piezoelectric micromachined ultrasonic transducer of claim 1, wherein the material of the structural layer is silicon.
5. The multi-frequency piezoelectric micromachined ultrasonic transducer of claim 1, wherein the insulating layer is silicon dioxide.
6. A method for preparing a multi-frequency piezoelectric micromechanical ultrasonic transducer, which is used for preparing the multi-frequency piezoelectric micromechanical ultrasonic transducer according to any one of claims 1-5, the method for preparing the multi-frequency piezoelectric micromechanical ultrasonic transducer comprising:
Etching two Helmholtz resonant cavities on a substrate and etching a communicated neck channel between the two Helmholtz resonant cavities;
Preparing a structural layer on the etched substrate by using a vapor deposition method;
Sequentially growing a bottom electrode layer, a piezoelectric layer and a top electrode layer on the structural layer by utilizing a magnetron sputtering process;
etching the piezoelectric layer and the top electrode layer by using a plasma etching method;
And depositing an insulating layer on the surface of the bottom electrode layer to obtain the multi-frequency piezoelectric micromechanical ultrasonic transducer.
7. The method for preparing the multi-frequency piezoelectric micromachined ultrasonic transducer according to claim 6, wherein the preparing the structural layer on the etched substrate by using a vapor deposition method specifically comprises:
utilizing a wet silicon oxide layer for the etched substrate;
Preparing polycrystalline silicon on the oxidized silicon layer by using a vapor deposition method as a sacrificial layer and polishing the surface of the sacrificial layer;
And preparing a structural layer on the sacrificial layer after surface polishing by using a vapor deposition method and carrying out surface polishing on the structural layer.
8. The method of manufacturing a multi-frequency piezoelectric micromachined ultrasonic transducer according to claim 7, further comprising, after the sequentially growing a bottom electrode layer, a piezoelectric layer, and a top electrode layer on the structural layer using a magnetron sputtering process:
etching the through hole to the sacrificial layer by using a dry method;
etching the sacrificial layer to the piezoelectric layer by utilizing xenon difluoride solution to obtain a back cavity structure combined with the neck channel;
And filling the through holes.
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| PCT/CN2023/115748 WO2024078171A1 (en) | 2022-10-14 | 2023-08-30 | Multi-frequency piezoelectric micromachined ultrasonic transducer and manufacturing method |
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| CN117750285B (en) * | 2024-02-07 | 2024-05-07 | 山东博华电子科技发展有限公司 | Preparation method of miniature piezoelectric transducer and miniature piezoelectric transducer |
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| WO2024078171A1 (en) | 2024-04-18 |
| CN115532572A (en) | 2022-12-30 |
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