CN115432662B - Micromechanical ultrasonic transducer with central support bottom electrode - Google Patents
Micromechanical ultrasonic transducer with central support bottom electrode Download PDFInfo
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- CN115432662B CN115432662B CN202210972097.7A CN202210972097A CN115432662B CN 115432662 B CN115432662 B CN 115432662B CN 202210972097 A CN202210972097 A CN 202210972097A CN 115432662 B CN115432662 B CN 115432662B
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B81—MICROSTRUCTURAL TECHNOLOGY
- B81B—MICROSTRUCTURAL DEVICES OR SYSTEMS, e.g. MICROMECHANICAL DEVICES
- B81B7/00—Microstructural systems; Auxiliary parts of microstructural devices or systems
- B81B7/02—Microstructural systems; Auxiliary parts of microstructural devices or systems containing distinct electrical or optical devices of particular relevance for their function, e.g. microelectro-mechanical systems [MEMS]
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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
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B81—MICROSTRUCTURAL TECHNOLOGY
- B81B—MICROSTRUCTURAL DEVICES OR SYSTEMS, e.g. MICROMECHANICAL DEVICES
- B81B7/00—Microstructural systems; Auxiliary parts of microstructural devices or systems
- B81B7/0032—Packages or encapsulation
- B81B7/007—Interconnections between the MEMS and external electrical signals
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- 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
- B81C1/00134—Manufacture or treatment of devices or systems in or on a substrate for manufacturing microsystems comprising flexible or deformable structures
- B81C1/00158—Diaphragms, membranes
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- 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
- B81C1/00261—Processes for packaging MEMS devices
- B81C1/00301—Connecting electric signal lines from the MEMS device with external electrical signal lines, e.g. through vias
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B81—MICROSTRUCTURAL TECHNOLOGY
- B81B—MICROSTRUCTURAL DEVICES OR SYSTEMS, e.g. MICROMECHANICAL DEVICES
- B81B2201/00—Specific applications of microelectromechanical systems
- B81B2201/03—Microengines and actuators
- B81B2201/032—Bimorph and unimorph actuators, e.g. piezo and thermo
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- Manufacturing & Machinery (AREA)
- Computer Hardware Design (AREA)
- Mechanical Engineering (AREA)
- Transducers For Ultrasonic Waves (AREA)
Abstract
The invention belongs to the technical field of micro-machinery, and particularly relates to a micro-mechanical ultrasonic transducer with a central support bottom electrode. The structure of the invention comprises the following components from bottom to top: a substrate, a center support, an edge support, a cavity, a flexible bottom electrode, a diaphragm, and a top electrode. The flexible bottom electrode is anchored to the substrate by a central support post or wall, and the diaphragm is secured to the substrate by an edge support, with a cavity between the diaphragm and the flexible bottom electrode. Under electrostatic interaction, the diaphragm and the flexible bottom electrode can both deform significantly. The micromechanical ultrasonic transducer of the present invention may be fabricated by either a sacrificial layer release process or a wafer bonding process. Compared with the capacitive micro-mechanical transducer with the traditional structure, the micro-mechanical ultrasonic transducer can realize higher ultrasonic transmitting sound pressure and receiving sensitivity. Meanwhile, the structure of the invention shows more remarkable spring softening effect and can be suitable for the environment needing to dynamically change the height or the center frequency of the cavity.
Description
Technical Field
The invention belongs to the technical field of micro-machinery, and particularly relates to a micro-mechanical ultrasonic transducer.
Background
Traditional biomedical imaging techniques such as magnetic resonance imaging, computed tomography and positron emission tomography are not suitable for repeated use on the human body for reasons of ionizing radiation, patient transfer, high cost, cumbersome equipment, and the like. Unlike most clinical biomedical imaging techniques, ultrasound is a very safe, free of ionizing radiation, relatively inexpensive, available for portable and real-time screening, and widely applicable to global clinical environments. The method has simple working principle, relates to sound wave reflection, and can provide an anatomic map of an imaging tissue area.
Ultrasonic sensing technology is highly advantageous due to the advent of microelectromechanical systems (MEMS). This is mainly because MEMS technology allows sensor miniaturization and integration. An ultrasonic transceiver integrated sensor based on micromachining is commonly referred to as a micromachined ultrasonic transducer (MicromachinedUltrasonicTransducer, MUT). MUTs are generally characterized by a membrane structure located above a cavity that generates ultrasonic wave-dependent bending vibrations of the membrane. In its emission mode, the vibrating membrane can generate high-frequency vibrations due to piezoelectric or electrostatic effects, and mechanical energy is transferred to the medium adjacent thereto, generating ultrasonic waves. In its receiving mode, the acoustic energy of the ultrasonic waves propagating in the medium in which the transducer is placed causes the membrane to vibrate, converting it into mechanical energy, and thus generating an electrical signal that is easily detected.
Among several types of MUTs, piezoelectric micromachined ultrasonic transducers (PiezoelectricMicromachinedUltrasonic Transducer, PMUT) driven using a piezoelectric effect and capacitive micromachined ultrasonic transducers (CAPACITIVE MICROMACHINED ULTRASONIC TRANSDUCER, CMUT) driven using an electrostatic force are widely studied. Compared with widely used lead zirconate titanate piezoelectric ceramic (PZT) ultrasonic transducers, the current MUT structure still has to realize breakthrough of bottleneck of transmitting sound pressure and receiving sensitivity.
Disclosure of Invention
The invention aims to provide a Micromechanical Ultrasonic Transducer (MUT) with a central support bottom electrode, so as to improve the transmitting sound pressure and receiving sensitivity of the MUT and obtain a larger central frequency adjustable range.
The micromechanical ultrasonic transducer provided by the invention is a two-dimensional array formed by continuation of micromechanical ultrasonic transducer units; the micromechanical ultrasonic transducer unit structure comprises the following components from top to bottom: a substrate 1, a center support 2, an edge support 3, a cavity 4, a flexible bottom electrode 5, a diaphragm 6 and a top electrode 7, see fig. 1 and 2. Wherein:
The center support 2 and the edge support 3 are fixed on the substrate; an edge support 3 is located around the edge of the substrate for supporting the diaphragm 6; a center support 2 is located at a center portion of the substrate for supporting the flexible bottom electrode 5;
the cavity 4 is formed by surrounding the edge support 3 and the vibrating membrane 6, the flexible bottom electrode 5 is positioned in the cavity 4, the edge of the flexible bottom electrode is spaced from the edge support 3, and the plane of the flexible bottom electrode is spaced from the vibrating membrane 6; namely, a motion (vibration) space is reserved between the upper and lower parts of the bending bottom electrode 5 and the vibrating membrane 6;
The top electrode 7 is positioned above the vibrating membrane 6, and the area of the top electrode is generally smaller than or equal to the vibrating membrane 6; the bendable bottom electrode 5 and the top electrode 7 form an electrode pair;
The deformations of the top electrode 7 and the diaphragm 6 are substantially synchronized.
The ultrasonic transducer designed by the invention has the working principle that:
A dc bias voltage is applied between the flexible bottom electrode 5 and the top electrode 7, which is subjected to electrostatic forces. Under the action of electrostatic force, the flexible bottom electrode 5 and the diaphragm 6 will deform, reducing the distance between each other. An alternating small signal voltage is applied between the flexible bottom electrode 5 and the top electrode 7, and the flexible bottom electrode 5 and the diaphragm 6 will vibrate. This vibration will result in the generation of sound waves. Conversely, an external acoustic wave may also cause vibration of the diaphragm 6, thereby changing the distance between the curved bottom electrode 5 and the diaphragm 6, resulting in a detectable change in the electrical signal. Thereby, the transducer achieves a mutual conversion of acoustic energy and electrical energy.
Further, in the present invention:
The transducer unit is circular or rectangular in its entirety, and accordingly, its constituent parts, the substrate 1, the center support 2, the flexible bottom electrode 5, the diaphragm 6, and the top electrode 7, are all circular or rectangular.
The substrate 1 is typically 200-500 μm thick to provide sufficient mechanical support, and is typically borosilicate glass or silicon.
The center support 2 and the flexible bottom electrode 5 are typically comprised of conductively doped silicon. The height of the center support 2 is usually 1-5 μm, and the shape is a cylinder with a radius of 10-50 μm or a rectangular wall with a length of 20-100 μm and a width of 2-10 μm. The flexible bottom electrode 5 is typically 1-5 μm thick, in the shape of a circular membrane with a radius of 10-50 μm or a square membrane 20-100 μm long and 20-100 μm wide, often with dimensions larger than the center support 2.
The edge support 3 and the diaphragm 6 are typically made of silicon or silicon dioxide. The edge support 3 has an inner diameter larger than the flexible bottom electrode 5, typically 10-50 μm, and a diaphragm 6 on top thereof, and a gap height with the flexible bottom electrode 5, typically 0.01-1 μm, to provide a larger effective capacitance. The thickness of the diaphragm 6 is usually 1 to 5 μm.
The top electrode 7 is usually metal, and has a radius equal to or smaller than that of the diaphragm 6 and a thickness of 300-500nm.
The flexible bottom electrode 5 is anchored to the substrate 1 by a central support 2, the edges of which can be deformed considerably. The diaphragm 6 is bound by the edge support 3, and its deformation is mainly in the central part.
The micromechanical ultrasonic transducer of the present invention needs to apply a dc bias voltage during operation, resulting in bending of the diaphragm 6 and the flexible bottom electrode 5, with reduced spacing. Due to the flexibility of the bottom electrode, the structure provided by the invention can realize smaller cavity height under the same direct current bias voltage, thereby realizing high-performance ultrasonic signal receiving and transmitting.
The invention solves the problems of small transmitting sound pressure and low receiving sensitivity of the micromechanical ultrasonic transducer with the traditional structure. When the micro-mechanical ultrasonic transducer with the center supporting the bottom electrode transmits and receives ultrasonic signals, the bottom electrode is free from side boundary constraint, deformation and vibration can occur, and the movement amplitude of the edge of the micro-mechanical ultrasonic transducer is larger. In this vibration mode, the diaphragm will be subjected to a greater electrostatic force to achieve a higher ultrasonic transceiving efficiency.
The micro-mechanical ultrasonic transducer provided by the invention can effectively dynamically change the height of the cavity due to the bendable bottom electrode, and simultaneously shows more remarkable spring softening effect: the center frequency varies more with the dc bias voltage. Therefore, the micromechanical ultrasonic transducer can realize real-time adjustment of the cavity height and the center frequency, and is suitable for more complex operation environments.
In addition, the micromachined ultrasonic transducer of the present invention can realize ultrasonic transmission and reception by vibration of the bottom electrode even when operation is continued while the upper deflectable membrane is hindered from vibration (for example, when the transducer is pressed against a solid surface such as the skin of a patient).
In addition to diaphragm dimensions in conventional designs, the dimensions of the center support and the flexible bottom electrode will also affect the performance characteristics of the transducer, such as center frequency, operating voltage, etc. Therefore, the micromechanical ultrasound transducer of the present invention has an additional degree of freedom of design. By designing the shape and size of different center supports and flexible bottom electrodes, transducers with different center frequencies and working voltages can be realized without changing parameters such as the height of the vibrating membrane and the cavity.
The invention provides a micromechanical ultrasonic transducer with a central support bottom electrode, which also comprises a deformation structure I. Specifically, the constituent materials of the substrate 1, the center support 2, and the flexible bottom electrode 5 are all conductively doped silicon, as shown in fig. 5. The structure is typically processed by a wafer bonding process.
The invention provides a micromechanical ultrasonic transducer with a central support bottom electrode, which also comprises a deformation structure II. Specifically, the center support 2 is a circular column with a hollowed center. The annular ring is typically 2-10 μm thick and has an outer diameter smaller than the radius of the flexible bottom electrode 5, as shown in fig. 6. The central support 2 may also be a centrally symmetrical two or more parallel rows of rectangular parallelepiped walls. The annular support structure has better mechanical robustness.
The invention provides a micromechanical ultrasonic transducer with a central support bottom electrode, which also comprises a deformation structure III. Specifically, an insulating substrate 1-1 and a substrate electrode 1-2 together form a conductive substrate 1; the flexible bottom electrode 5 is composed of a bottom electrode insulating layer 5-1 and a bottom electrode layer 5-2, as shown in fig. 7. The structure has three electrode layers: a substrate electrode 1-2, a bottom electrode 5-2 and a top electrode 7. The structure can realize electrostatic shielding by grounding the substrate electrode 1-2 and the top electrode 7 on one hand, thereby improving electrical safety, and can also utilize the substrate electrode 1-2 as an electrostatic electrode on the other hand, so that the flexible bottom electrode 5 can be deformed more flexibly.
The invention provides a micromechanical ultrasonic transducer with a central support bottom electrode, which also comprises a deformation structure IV. Specifically, the flexible bottom electrode 5 is composed of an elastic layer (lower electrode insulating layer) 5-1, a lower electrode layer 5-2, a piezoelectric layer 5-3, and an upper electrode layer 5-4. By the piezoelectric effect, the morphology of the flexible bottom electrode 5 can be adjusted, as shown in fig. 8.
The invention provides a micromechanical ultrasonic transducer with a central support bottom electrode, which also comprises a deformation structure V. Specifically, the top electrode 7 is composed of a lower electrode layer 7-1, a piezoelectric layer 7-2, and an upper electrode layer 7-3. The form of the diaphragm 6 can be adjusted by the piezoelectric effect, as shown in fig. 9.
The invention also provides a processing method of the micro-mechanical ultrasonic transducer, which comprises a sacrificial layer release method, as shown in fig. 10, and comprises the following specific steps:
step 1, preparing a substrate wafer;
Step 2, depositing a first sacrificial layer on the substrate wafer, and performing graphical etching;
Step 3, depositing a conductive doped center support and a bottom electrode layer;
step 4, depositing a second sacrificial layer and performing graphical etching;
step 5, depositing an edge support and a vibrating membrane;
step 6, releasing the first sacrificial layer and the second sacrificial layer through corrosive liquid to form a cavity;
step 7, depositing a top electrode and performing graphical etching; finally, the micromechanical ultrasonic transducer unit with the bottom electrode supported at the center is obtained.
The processing method of the micro-mechanical ultrasonic transducer provided by the invention further comprises a wafer bonding method, as shown in fig. 11, and specifically comprises the following steps:
Step 1, preparing a first substrate wafer;
step 2, etching a cavity on the first substrate wafer;
Step 3, preparing a polished second SOI wafer which comprises a substrate silicon layer, a buried oxide layer and a device layer; bonding the first wafer and the second SOI wafer in the step 2;
step 4, removing the substrate silicon layer and the buried oxide layer of the second SOI wafer;
Step 5, etching the device layer to expose the cavity;
Step 6, preparing a third SOI sheet of a thick device layer, wherein the third SOI sheet comprises a substrate silicon layer, a buried oxide layer and a device layer;
Step 7, etching the third SOI wafer in a graphical way;
Step 8, bonding the third SOI wafer with the wafer obtained in the step 5;
Step 9, removing the substrate silicon layer and the buried oxide layer of the third SOI wafer;
step 10, depositing a top electrode and performing graphical etching; finally, the micromechanical ultrasonic transducer unit with the bottom electrode supported at the center is obtained.
Fig. 3 shows a schematic diagram of the deformation of the micromechanical ultrasonic transducer under a dc bias voltage. The diaphragm 6 is bound by the edge support 3, the edge is fixed, and the center deformation becomes dominant. The flexible bottom electrode 5 is anchored by the central support 2, fixed centrally, and the edge deformation is dominant.
Fig. 4 shows a schematic diagram of the deformation of a micromechanical ultrasonic transducer of conventional structure under a dc bias voltage. The diaphragm 6 is bound by the edge support 3, the edge is fixed, and the center deformation becomes dominant. The bottom electrode 5 is fixed on the substrate 1 without deformation.
Fig. 12 and 13 show the results of static deformation simulation of the vibrating layer and the bottom electrode of the micromechanical ultrasonic transducer with the center post supporting the bottom electrode and the micromechanical ultrasonic transducer with a conventional structure, respectively. By comparison, it can be seen that the vibrating membrane of the micromechanical transducer provided by the invention has larger static deformation.
Figures 14, 15 and 16 show the mean displacement of the diaphragms, fractional capacitance variation and resonant frequency contrast of the micromechanical ultrasonic transducer with the central column supporting the bottom electrode and the micromechanical ultrasonic transducer with conventional structure, respectively. It can be seen that the micromachined ultrasonic transducer exhibits more pronounced diaphragm average displacement, fractional capacitance change, and resonant frequency change as the dc bias voltage increases.
Fig. 17 and 18 show the mean vibration amplitude of the diaphragm and the far-field sound pressure contrast of the micromechanical ultrasonic transducer with the center pillar supporting the bottom electrode and the micromechanical ultrasonic transducer with the conventional structure, respectively. It can be seen that the average amplitude peak and far-field sound pressure peak of the micromechanical ultrasonic transducer diaphragm are much larger than those of the conventional structure.
Drawings
Fig. 1 is a schematic diagram of the basic structure of a MUT with a center pillar supporting a bottom electrode according to the present invention.
Fig. 2 is a schematic diagram of the basic structure of a MUT with a bottom electrode supported by a center wall according to the present invention.
Fig. 3 is a schematic diagram of static deformation of a MUT with a center support bottom electrode according to the present invention.
Fig. 4 is a schematic diagram of static deformation of a MUT with a conventional structure.
Fig. 5 shows a modification of the MUT structure of the center support bottom electrode of the present invention.
Fig. 6 shows a second modification of the MUT structure of the center support bottom electrode of the present invention.
Fig. 7 shows a modification of the MUT structure of the center support bottom electrode of the present invention.
Fig. 8 shows a modification of the MUT structure of the center support bottom electrode of the present invention.
Fig. 9 is a modified MUT structure five of the center support bottom electrode of the present invention.
FIG. 10 is a flow chart of a sacrificial layer release process for a MUT with a center support bottom electrode according to the present invention.
Fig. 11 is a wafer bonding process flow diagram of a MUT with a center support bottom electrode in accordance with the present invention.
Fig. 12 is a simulation result of static deformation of the vibrating layer and bottom electrode of the MUT with the center post supporting the bottom electrode of the present invention.
Fig. 13 is a simulation result of static deformation of the vibrating layer and the bottom electrode of the MUT with the conventional structure.
Fig. 14 is a simulation result comparing the relationship between the average displacement of the diaphragm of the MUT of the center pillar support bottom electrode and the conventional structure MUT according to the dc bias voltage.
Fig. 15 is a simulation result comparing fractional capacitance variation of the MUT of the center pillar support bottom electrode of the present invention and the MUT of the conventional structure with dc bias voltage variation.
Fig. 16 is a simulation result comparing the relationship between the resonance frequency of the MUT of the center pillar support bottom electrode and the conventional structure MUT according to the dc bias voltage.
Fig. 17 is a simulation result comparing the average amplitude of the diaphragm of the MUT of the center pillar support bottom electrode of the present invention and the MUT of the conventional structure.
Fig. 18 is a simulation result comparing far field emission sound pressure of MUT of the center pillar supporting bottom electrode of the present invention and MUT of conventional structure.
Detailed Description
The invention will be described in more detail below with reference to the accompanying drawings. Like elements are denoted by like reference numerals throughout the various drawings. For clarity, the various features of the drawings are not drawn to scale. Furthermore, some well known parts may not be shown.
Numerous specific details of the invention, such as device structures, material sizing processes, and techniques, are set forth below in order to provide a more thorough understanding of the invention. As will be understood by those skilled in the art, the present invention may be practiced without these specific details.
Fig. 1 is a schematic diagram showing the basic structure of a MUT having a center pillar supporting a bottom electrode according to the present invention.
Referring to the cross section and the top view in the figure, the MUT of the bottom electrode is supported by the center column according to the present invention, which comprises the following steps: a substrate 1, a center support 2, an edge support 3, a cavity 4, a flexible bottom electrode 5, a diaphragm 6 and a top electrode 7. The MUT transducer unit with the center post supporting the bottom electrode is circular in shape as a whole, and the constituent components of the substrate 1, the center support 2, the flexible bottom electrode 5, the diaphragm 6 and the top electrode 7 are all circular.
The center support 2 and the edge support 3 are fixed on the substrate and respectively support the flexible bottom electrode 5 and the vibrating membrane 6; the cavity 4 is surrounded by the edge support 3 and the vibrating membrane 6, and provides a space for the flexible bottom electrode 5 and the vibrating membrane 6 to move up and down; the bendable bottom electrode 5 is positioned inside the cavity 4; the top electrode 7 is located above the diaphragm 6 and has an area generally equal to or smaller than the diaphragm 6. The deformations of the top electrode 7 and the diaphragm 6 are substantially synchronized.
The substrate 1 has a thickness of 500 μm and is made of borosilicate glass. The central support 2 and the flexible bottom electrode 5 are typically made of conductively doped silicon. The center support 2 has a height of 1 μm and a radius of 2.5 μm. The thickness of the flexible bottom electrode 5 is 2 μm and the radius is 14 μm. The material of the edge support 3 and the diaphragm 6 is silicon. The edge support 3 is 15 μm and has a diaphragm 6 on top and a gap height of 100nm with the flexible bottom electrode 5. The thickness of the diaphragm 6 was 1.5. Mu.m. The material of the top electrode 7 is gold with a radius of 10 μm and a thickness of 300nm. The flexible bottom electrode 5 is anchored to the substrate 1 by the central support 2, the edges of which can be deformed considerably. The diaphragm 6 is bounded by the edge support 3 and its deformation is centered.
Fig. 2 is a schematic diagram showing the basic structure of the MUT of the center wall support bottom electrode of the present invention.
Referring to the cross section and the top view in the figure, the MUT of the central wall supporting bottom electrode provided by the invention comprises the following components from bottom to top: a substrate 1, a center support 2, an edge support 3, a cavity 4, a flexible bottom electrode 5, a diaphragm 6 and a top electrode 7. The MUT transducer unit with the bottom electrode supported by the center wall is rectangular in whole, and the substrate 1, the center support 2, the flexible bottom electrode 5, the vibrating membrane 6 and the top electrode 7 which are component parts are rectangular.
The center support 2 and the edge support 3 are fixed on the substrate and respectively support the flexible bottom electrode 5 and the vibrating membrane 6; the cavity 4 is surrounded by the edge support 3 and the vibrating membrane 6, and provides a space for bending the bottom electrode 5 and the vibrating membrane 6 to move up and down; the bendable bottom electrode 5 is positioned inside the cavity 4; the top electrode 7 is located above the diaphragm 6 and has an area generally equal to or smaller than the diaphragm 6. The deformations of the top electrode 7 and the diaphragm 6 are substantially synchronized.
The substrate 1 has a thickness of 500 μm and is made of borosilicate glass. The central support 2 and the flexible bottom electrode 5 are typically made of conductively doped silicon. The center support 2 has a height of 1 μm and a radius of 2.5 μm. The thickness of the flexible bottom electrode 5 is 2 μm and the radius is 14 μm. The material of the edge support 3 and the diaphragm 6 is silicon. The edge support 3 is 15 μm and has a diaphragm 6 on top and a gap height of 100nm with the flexible bottom electrode 5. The thickness of the diaphragm 6 was 1.5. Mu.m. The material of the top electrode 7 is gold with a radius of 10 μm and a thickness of 300nm. The flexible bottom electrode 5 is anchored to the substrate 1 by the central support 2, the edges of which can be deformed considerably. The diaphragm 6 is bounded by the edge support 3 and its deformation is centered.
Fig. 3 shows a static deformation diagram of the MUT of the center support bottom electrode of the present invention.
As shown in fig. 3, with the micromechanical transducer of the present invention, the diaphragm 6 is bounded by the edge support 3, the edge is fixed, and the central deformation is dominant. The flexible bottom electrode 5 is anchored by the central support 2, fixed centrally, and the edge deformation is dominant. When a DC bias voltage is applied between the top electrode 7 and the flexible bottom electrode 5, the center of the diaphragm 6 is depressed downward, and the edges of the flexible bottom electrode 5 are warped upward, so that the distance between each other is reduced.
Fig. 4 shows a static deformation diagram of a MUT of conventional structure.
The diaphragm 6 is bound by the edge support 3, the edge is fixed, and the center deformation becomes dominant. When a DC bias voltage is applied between the top electrode 7 and the bottom electrode 5, the center of the diaphragm 6 is depressed, the bottom electrode 5 is not deformed, and the distance between the electrodes is reduced.
Fig. 5 shows a modification of the MUT structure of the center support bottom electrode of the present invention.
As shown in fig. 5, the first modified structure is characterized in that the constituent materials of the substrate 1, the center support 2 and the flexible bottom electrode 5 are all conductively doped silicon. The structure is typically processed by a wafer bonding process.
Fig. 6 shows a second modification of the MUT of the present invention with the bottom electrode centrally supported.
As shown in fig. 6, the second deformation structure is characterized in that the center support 2 is a circular column with a hollow center. The annular ring has a thickness of 3 μm and an outer diameter of 8 μm, which is smaller than the radius 14 μm of the flexible bottom electrode 5. The central support 2 may also be a centrally symmetrical two or more parallel rows of rectangular parallelepiped walls. The annular support structure has better mechanical robustness.
Fig. 7 shows a modification of the MUT structure three of the center support bottom electrode of the present invention.
As shown in fig. 7, the deformed structure three is characterized in that the insulating substrate 1-1 and the substrate electrode 1-2 together constitute the conductive substrate 1; the flexible bottom electrode 5 is composed of a bottom electrode insulating layer 5-1 and a bottom electrode layer 5-2. The structure has three electrode layers: a substrate electrode 1-2, a bottom electrode 5-2 and a top electrode 7. The structure can realize electrostatic shielding by grounding the substrate electrode 1-2 and the top electrode 7 on one hand, thereby improving electrical safety, and can also utilize the substrate electrode 1-2 as an electrostatic electrode on the other hand, so that the flexible bottom electrode 5 can be deformed more flexibly.
Fig. 8 shows a modification of the MUT structure of the center support bottom electrode of the present invention.
As shown in fig. 8, the modification structure four is characterized in that the flexible bottom electrode 5 is composed of an insulating layer (elastic layer) 5-1, a lower electrode layer 5-2, a piezoelectric layer 5-3, and an upper electrode layer 5-4. The morphology of the flexible bottom electrode 5 can be adjusted by the piezoelectric effect.
Fig. 9 shows a modification five of the MUT of the present invention with the bottom electrode centrally supported.
As shown in fig. 9, the modification structure five is characterized in that the top electrode 7 is composed of a lower electrode layer 7-1, a piezoelectric layer 7-2, and an upper electrode layer 7-3. The form of the diaphragm 6 can be adjusted by the piezoelectric effect.
Fig. 10 shows a sacrificial layer release process flow diagram of the MUT of the present invention with a center support bottom electrode.
As shown in fig. 10, the sacrificial layer release process flow of the MUT with the bottom electrode supported at the center includes the following steps:
Step 1, preparing a first substrate wafer, wherein the material of the first substrate wafer is borosilicate glass;
And 2, depositing a first sacrificial layer on the first substrate wafer, and performing graphical etching to define the graph of the cavity.
Step 3, depositing a conductive doped center support and a bottom electrode layer, wherein the material is silicon;
Step 4, depositing a second sacrificial layer and carrying out graphical etching on the edge of the second sacrificial layer;
Step 5, depositing an edge support and a vibrating membrane, wherein the material is silicon;
step 6, injecting a specific corrosive liquid into the etching holes to corrode the first sacrificial layer and the second sacrificial layer, so as to form cavities;
and 7, depositing gold, and performing graphical etching to form a top electrode. Finally, a MUT cell with a center supporting the bottom electrode is obtained.
Fig. 11 is a wafer bonding process flow diagram of a MUT with a center support bottom electrode in accordance with the present invention.
As shown in fig. 11, the wafer bonding process flow of the MUT with the bottom electrode supported at the center includes the following steps:
step 1, preparing a first substrate wafer, wherein the material is silicon;
Step 2, carrying out graphical etching on the first substrate wafer to define a cavity;
Step 3, preparing a polished second SOI wafer which comprises a substrate silicon layer, a buried oxide layer and a device layer; bonding the first wafer and the second SOI wafer in the step 2;
step 4, removing the substrate silicon layer 2-1 and the buried oxide layer 2-2 of the second SOI wafer;
step 5, etching the device layer 2-1 to expose the cavity 4;
Step 6, preparing a third SOI sheet of a thick device layer, wherein the third SOI sheet comprises a substrate silicon layer, a buried oxide layer and a device layer;
Step 7, patterning and etching the device layer of the third SOI wafer, and taking the thickness of 1.5 mu m as a vibrating membrane;
Step 8, bonding the third SOI wafer with the wafer obtained in the step 5;
step 9, removing the substrate silicon layer and the buried oxide layer of the third SOI wafer 3;
step 10, depositing gold, and performing graphical etching to form a top electrode; finally, a MUT cell with a center supporting the bottom electrode is obtained.
Fig. 12 shows the results of static deformation simulation of the vibrating layer and bottom electrode of the MUT of the invention with the center post supporting the bottom electrode.
As shown in fig. 12, the vibrating membrane (solid line in the figure) of the MUT of the center pillar supporting the bottom electrode of the present invention has a fixed edge, a depressed center, and a flexible bottom electrode (broken line in the figure) has a fixed center and a tilted edge when different dc bias voltages are applied. The distance between the two is reduced.
Fig. 13 shows the results of static deformation simulation of the vibrating layer and bottom electrode of the MUT of the conventional structure.
As shown in fig. 13, in the case of applying different dc bias voltages, the diaphragm (solid line in the figure) of the MUT of the conventional structure is fixed at the edge, with the center recessed downward, and the bottom electrode (broken line in the figure) is fixed without deformation. The distance between the two is reduced.
Fig. 14 shows a comparison of simulation results of the relationship between the average displacement of the diaphragm of the MUT of the center post supporting bottom electrode and the MUT of the conventional structure according to the dc bias voltage.
As shown in fig. 14, the MUT of the center pillar support bottom electrode of the present invention exhibits a larger diaphragm deformation than the MUT of the conventional structure, and this difference is more pronounced as the dc bias voltage increases. This is because the electrostatic force effect due to the upward warp of the flexible bottom electrode is enhanced.
Fig. 15 shows a comparison of simulation results of fractional capacitance change of the MUT of the center pillar support bottom electrode and the MUT of the conventional structure according to dc bias voltage change.
For the static capacitance value of the MUT, the fractional capacitance change is defined as the capacitance at dc bias voltage divided by the percentage of capacitance at no dc bias voltage. As shown in fig. 15, the MUT of the center pillar support bottom electrode of the present invention exhibits a larger fractional capacitance variation, and this difference is more pronounced as the dc bias voltage increases. This is mainly due to the fact that the flexible bottom electrode is warped upward, which results in a further decrease in the inter-electrode distance.
Fig. 16 shows a comparison of simulation results of the relationship between the resonance frequency of the MUT of the center pillar support bottom electrode and the conventional structure MUT according to the dc bias voltage.
The CMUT exhibits a spring softening effect due to a nonlinear effect of electrostatic force, i.e., the resonance frequency decreases with an increase in dc bias voltage. As shown in fig. 16, the resonance frequency of the MUT of the center pillar support bottom electrode of the present invention is more significantly shifted than that of the MUT of the conventional structure. This indicates that its spring softening effect is more pronounced.
Fig. 17 shows a comparison of simulation results of the mean amplitude of the diaphragm of the MUT of the center post support bottom electrode of the present invention and the MUT of the conventional structure.
As shown in FIG. 17, the frequency characteristics of the average amplitude of the diaphragm under the excitation of 10-20MHz AC small signal were analyzed by simulation for MUT of the center pillar support bottom electrode of the present invention and MUT of conventional structure. Compared with the MUT with the traditional structure, the average amplitude peak value of the MUT of the central column supporting bottom electrode is about 240nm, which is far greater than 80nm of the MUT with the traditional structure.
Fig. 18 shows a comparison of simulation results of far-field emission sound pressure of the MUT of the center pillar support bottom electrode of the present invention and the MUT of the conventional structure.
As shown in FIG. 18, the frequency characteristics of far-field emission sound pressure of the MUT of the center pillar supporting bottom electrode and the MUT of the traditional structure under the excitation of 10-20MHz alternating current small signal are simulated and analyzed. Compared with the MUT with the traditional structure, the peak sound pressure of the MUT with the central column supporting the bottom electrode at the distance of 3mm is about 1100Pa, and is far greater than that of the MUT with the traditional structure. This illustrates that the MUT of the center pillar support bottom electrode of the present invention has a higher emitted sound pressure. .
Many variations and modifications may be made by one of ordinary skill in the art in light of the disclosure herein without departing from the spirit and scope of the invention, which is also to be considered as being within the scope of the invention.
Claims (9)
1. The micromechanical ultrasonic transducer with the bottom electrode supported at the center is characterized by being a two-dimensional array formed by continuation of micromechanical ultrasonic transducer units; the micromechanical ultrasonic transducer unit structure comprises the following components from bottom to top: a substrate (1), a center support (2), an edge support (3), a cavity (4), a flexible bottom electrode (5), a vibrating membrane (6) and a top electrode (7); wherein:
The center support (2) and the edge support (3) are fixed on the substrate (1); an edge support (3) is positioned around the edge of the substrate for supporting the diaphragm (6); a center support (2) located at a center portion of the substrate for supporting the flexible bottom electrode (5);
the cavity (4) is formed by encircling the edge support (3) and the vibrating membrane (6), the bendable bottom electrode (5) is positioned in the cavity (4), the edge of the bendable bottom electrode is spaced from the edge support (3), and a space is reserved between the plane of the bendable bottom electrode and the vibrating membrane (6); namely, the bendable bottom electrode (5) and the vibrating membrane (6) are provided with a movement space up and down;
The top electrode (7) is positioned above the vibrating membrane (6) and has an area smaller than or equal to the vibrating membrane (6); the bendable bottom electrode (5) and the top electrode (7) form an electrode pair;
Applying a dc bias voltage between the flexible bottom electrode (5) and the top electrode (7) will create an electrostatic force effect; under the action of electrostatic force, the flexible bottom electrode (5) and the vibrating membrane (6) deform, and the distance between the flexible bottom electrode and the vibrating membrane is reduced; applying an alternating small signal voltage between the flexible bottom electrode (5) and the top electrode (7) to cause the flexible bottom electrode (5) and the vibrating membrane (6) to vibrate; the vibration will result in the generation of sound waves; conversely, external sound waves can also cause vibration of the vibrating membrane (6), thereby changing the distance between the curved bottom electrode (5) and the vibrating membrane (6), producing a detectable change in the electrical signal; thereby, a mutual conversion of acoustic energy and electric energy is achieved.
2. Micromechanical ultrasonic transducer according to claim 1, characterized in that the transducer unit is entirely circular or rectangular, and that its component parts, the substrate (1), the center support (2), the flexible bottom electrode (5), the diaphragm (6) and the top electrode (7), respectively, are all circular or rectangular.
3. Micromechanical ultrasonic transducer according to claim 1, characterized in that the substrate (1) is typically 200-500 μm thick to provide sufficient mechanical support, the material of which is borosilicate glass or silicon.
4. Micromechanical ultrasonic transducer according to claim 1, characterized in that the central support (2) and the bendable bottom electrode (5) are made of conductively doped silicon; the height of the center support (2) is 1-5 mu m, and the shape of the center support is a cylinder with the radius of 10-50 mu m or a cuboid wall with the length of 20-100 mu m and the width of 2-10 mu m; the thickness of the flexible bottom electrode (5) is 1-5 mu m, the shape of the flexible bottom electrode is a round film with the radius of 10-50 mu m or a square film with the length of 20-100 mu m and the width of 20-100 mu m, and the size of the flexible bottom electrode is usually larger than that of the center support (2).
5. Micromechanical ultrasonic transducer according to claim 1, characterized in that the edge support (3) and the vibrating membrane (6) are made of silicon or silicon dioxide; the inner diameter of the edge support (3) is larger than that of the flexible bottom electrode (5) and is 10-50 mu m, the top of the edge support is provided with a vibrating membrane (6), and the height of a gap between the edge support and the flexible bottom electrode (5) is 0.01-1 mu m; the thickness of the diaphragm (6) is 1-5 mu m.
6. Micromechanical ultrasonic transducer according to claim 1, characterized in that the top electrode (7) is metal with a radius smaller than or equal to the diaphragm (6) and a thickness of 300-500nm.
7. Micromechanical ultrasonic transducer according to claim 1, characterized in that the flexible bottom electrode (5) is anchored to the substrate (1) by a central support (2), the edges of which are substantially deformable; the vibrating diaphragm (6) is bound by the edge support (3), and the deformation of the vibrating diaphragm is mainly centered.
8. Micromechanical ultrasonic transducer according to one of the claims 1-6, characterized in that it further comprises the following structural deformations:
Deformation structure one: the substrate (1), the center support (2) and the bendable bottom electrode (5) are all made of conductive doped silicon;
and a deformation structure II: the center support (2) is a circular column with a hollow center; the thickness of the circular ring is 2-10 mu m, and the outer diameter is smaller than the radius of the bendable bottom electrode (5); or the center support (2) is two or more rows of parallel cuboid walls which are symmetrical in center;
And a deformation structure III: the insulating substrate and the substrate electrode form a conductive substrate (1); the bendable bottom electrode (5) consists of a bottom electrode insulating layer and a bottom electrode layer; the structure has three electrode layers: a substrate electrode, a bottom electrode and a top electrode (7);
And a deformation structure IV: the bendable bottom electrode (5) consists of an elastic layer, a lower electrode layer, a piezoelectric layer and an upper electrode layer; the form of the bendable bottom electrode (5) is adjusted through the piezoelectric effect;
And a deformation structure five: the top electrode (7) consists of a lower electrode layer (7-1), a piezoelectric layer (7-2) and an upper electrode layer (7-3); the form of the diaphragm (6) is adjusted by the piezoelectric effect.
9. The method for manufacturing a micromechanical ultrasonic transducer according to any of claims 1-8, characterized in that (a) a sacrificial layer release process is used, comprising the following specific steps:
step 1, preparing a substrate wafer;
Step 2, depositing a first sacrificial layer on the substrate wafer, and performing graphical etching;
Step 3, depositing a conductive doped center support and a base electrode layer;
step 4, depositing a second sacrificial layer and performing graphical etching;
step 5, depositing an edge support and a vibrating membrane;
step 6, releasing the first sacrificial layer and the second sacrificial layer through corrosive liquid to form a cavity;
step 7, depositing a top electrode and performing graphical etching; finally, a micromechanical ultrasonic transducer unit with a central support bottom electrode is obtained;
or (II) adopting a wafer bonding process, which comprises the following specific steps:
Step 1, preparing a first substrate wafer;
step 2, etching a cavity on the first substrate wafer;
step 3, preparing a polished second SOI wafer which comprises a substrate silicon layer, a buried oxide layer and a device layer; bonding the first substrate wafer and the second SOI wafer in the step 2;
step 4, removing the substrate silicon layer and the buried oxide layer of the second SOI wafer;
Step 5, etching the device layer to expose the cavity;
step 6, preparing a second SOI wafer of a thick device layer, wherein the second SOI wafer comprises a substrate silicon layer, a buried oxide layer and a device layer;
Step 7, etching the third SOI wafer in a graphical way;
Step 8, bonding the third SOI wafer with the wafer obtained in the step 5;
Step 9, removing the substrate silicon layer and the buried oxide layer of the third SOI wafer;
step 10, depositing a top electrode and performing graphical etching; finally, the micromechanical ultrasonic transducer unit with the bottom electrode supported at the center is obtained.
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| CN102498586A (en) * | 2009-09-21 | 2012-06-13 | 香港理工大学 | Flexible capacitive micromachined ultrasonic transducer array with increased effective capacitance |
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