CN115432662B - Micromachined ultrasonic transducer with centrally supported bottom electrode - Google Patents

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

Micromachined ultrasonic transducer with centrally supported bottom electrode

Technical Field

The invention belongs to the field of micro-mechanical technology, and in particular relates to a micro-mechanical ultrasonic transducer.

Background Art

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 due to the use of ionizing radiation, the need to move the patient, high costs and bulky equipment. Unlike most clinical biomedical imaging techniques, ultrasound is a very safe, non-ionizing, relatively inexpensive, portable and real-time screening technology that is widely used in clinical settings around the world. Its working principle is simple, involving the reflection of sound waves, which provides an anatomical map of the imaged tissue area.

Ultrasonic sensing technology has benefited greatly from the emergence of microelectromechanical systems (MEMS). This is mainly because MEMS technology allows sensors to be miniaturized and integrated. Ultrasonic transceivers based on micromachining technology are usually called micromachined ultrasonic transducers (MUT). MUTs are usually characterized by a thin film structure located above a cavity, and the generation of ultrasonic waves depends on the bending vibration of the film. In its transmitting mode, the vibrating film can generate high-frequency vibrations due to piezoelectric or electrostatic effects through external electrical signal excitation, and the mechanical energy is transmitted to the adjacent medium to generate ultrasonic waves. In its receiving mode, the acoustic energy of the ultrasonic wave propagating in the medium where the transducer is placed causes the film to vibrate, which is converted into mechanical energy, and then an easily detectable electrical signal is generated.

Among several types of MUTs, piezoelectric micromachined ultrasonic transducers (PMUT) driven by piezoelectric effect and capacitive micromachined ultrasonic transducers (CMUT) driven by electrostatic force have been widely studied. Compared with the widely used lead zirconate titanate piezoelectric ceramic (PZT) ultrasonic transducers, the current MUT structure still needs to break through the bottlenecks of transmitting sound pressure and receiving sensitivity.

Summary of the invention

The object of the present invention is to provide a micromachined ultrasonic transducer (MUT) with a centrally supported bottom electrode, so as to improve the transmitting sound pressure and receiving sensitivity of the MUT and obtain a larger adjustable range of the central frequency.

The micromechanical ultrasonic transducer provided by the present invention is a two-dimensional array formed by extending a micromechanical ultrasonic transducer unit; the micromechanical ultrasonic transducer unit structure includes: a substrate 1, a central support 2, an edge support 3, a cavity 4, a flexible bottom electrode 5, a vibrating membrane 6 and a top electrode 7 arranged from top to bottom, as shown in Figures 1 and 2. Wherein:

The central support 2 and the edge support 3 are fixed on the substrate; the edge support 3 is located around the edge of the substrate and is used to support the vibration membrane 6; the central support 2 is located in the central part of the substrate and is used to support the flexible bottom electrode 5;

The cavity 4 is surrounded by the edge support 3 and the vibration membrane 6. The flexible bottom electrode 5 is located inside the cavity 4. There is a distance between its edge and the edge support 3, and there is a distance between its plane and the vibration membrane 6; that is, the flexible bottom electrode 5 and the vibration membrane 6 have movement (vibration) space both above and below.

The top electrode 7 is located above the vibration membrane 6, and its area is usually smaller than or equal to the vibration membrane 6; the bendable bottom electrode 5 and the top electrode 7 form an electrode pair;

The deformation of the top electrode 7 and the vibration membrane 6 are substantially synchronized.

The ultrasonic transducer designed by the present invention has the following working principle:

When a DC bias voltage is applied between the flexible bottom electrode 5 and the top electrode 7, an electrostatic force will be applied. Under the action of the electrostatic force, the flexible bottom electrode 5 and the vibrating membrane 6 will be deformed, and the distance between them will be reduced. When an AC small signal voltage is applied between the flexible bottom electrode 5 and the top electrode 7, the flexible bottom electrode 5 and the vibrating membrane 6 will vibrate. This vibration will lead to the generation of sound waves. Conversely, external sound waves can also cause the vibration of the vibrating membrane 6, thereby changing the distance between the flexible bottom electrode 5 and the vibrating membrane 6, and generating detectable electrical signal changes. In this way, the transducer realizes the mutual conversion of sound energy and electrical energy.

Further, in the present invention:

The transducer unit is circular or rectangular in shape as a whole, and accordingly, its components, the substrate 1 , the central support 2 , the flexible bottom electrode 5 , the vibration membrane 6 and the top electrode 7 are all circular or rectangular.

The thickness of the substrate 1 is usually 200-500 μm to provide sufficient mechanical support, and the substrate 1 is usually made of borosilicate glass or silicon.

The central support 2 and the flexible bottom electrode 5 are usually made of conductive doped silicon. The height of the central support 2 is usually 1-5 μm, and its 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 thickness of the flexible bottom electrode 5 is usually 1-5 μm, and its shape is a circular film with a radius of 10-50 μm or a square film with a length of 20-100 μm and a width of 20-100 μm, and its size is often larger than the central support 2.

The edge support 3 and the vibration membrane 6 are usually made of silicon or silicon dioxide. The inner diameter of the edge support 3 is larger than the flexible bottom electrode 5, usually 10-50 μm, and the top of the edge support 3 is the vibration membrane 6, and the gap height with the flexible bottom electrode 5 is usually 0.01-1 μm to provide a larger effective capacitance. The thickness of the vibration membrane 6 is usually 1-5 μm.

The top electrode 7 is usually made of metal, has a radius smaller than or equal to the vibration membrane 6, and a thickness of 300-500 nm.

The bendable bottom electrode 5 is anchored on the substrate 1 through the central support 2, and its edge can be deformed greatly. The vibration membrane 6 is bounded by the edge support 3, and its deformation is mainly in the center.

The micromechanical ultrasonic transducer of the present invention needs to apply a DC bias voltage when working, which causes the vibration membrane 6 and the flexible bottom electrode 5 to bend and reduce the spacing. Due to the bendability of the bottom electrode, the structure proposed by the present invention can achieve a smaller cavity height under the same DC bias voltage, thereby achieving high-performance ultrasonic signal transmission and reception.

The present invention solves the problem of low transmission sound pressure and low receiving sensitivity of the micromechanical ultrasonic transducer with a traditional structure. When the micromechanical ultrasonic transducer with a centrally supported bottom electrode transmits and receives ultrasonic signals, its bottom electrode has no side boundary constraints, can deform and vibrate, and its edge has a larger activity amplitude. In this vibration mode, the vibrating membrane will be subjected to a greater electrostatic force to achieve a higher ultrasonic transmission and reception efficiency.

Since the bottom electrode is bendable, the micromechanical ultrasonic transducer proposed by the present invention can effectively dynamically change the cavity height, and at the same time exhibits a more significant spring softening effect: the center frequency changes more with the DC bias voltage. Therefore, the micromechanical ultrasonic transducer of the present invention can achieve real-time adjustment of the cavity height and center frequency, and is suitable for more complex operating environments.

Additionally, the micromachined ultrasound transducer of the present invention can transmit and receive ultrasound through vibration of the bottom electrode even while continuing to operate when the upper deflectable membrane is prevented from vibrating (eg, when the transducer is pressed against a solid surface, such as a patient's skin).

In addition to the size of the vibrating membrane in the traditional structure, the size of the center support and the flexible bottom electrode will also affect the performance indicators of the transducer such as the center frequency and the operating voltage. Therefore, the micromechanical ultrasonic transducer of the present invention has additional design freedom. By designing different shapes and sizes of the center support and the flexible bottom electrode, transducers with different center frequencies and operating voltages can be realized without changing parameters such as the vibrating membrane and the cavity height.

The micromechanical ultrasonic transducer with a central support bottom electrode provided by the present invention also includes a deformation structure 1. Specifically, the constituent materials of the substrate 1, the central support 2 and the bendable bottom electrode 5 are all conductively doped silicon, as shown in Figure 5. This structure is generally processed by a wafer bonding process.

The micromechanical ultrasonic transducer with a central support bottom electrode provided by the present invention also includes a second deformation structure. Specifically, the central support 2 is a circular ring column with a hollow center. The thickness of the ring is usually 2 to 10 μm, and the outer diameter is smaller than the radius of the bendable bottom electrode 5, as shown in Figure 6. The central support 2 can also be two or more rows of parallel rectangular walls with central symmetry. This annular support structure has better mechanical robustness.

The micromechanical ultrasonic transducer with a centrally supported bottom electrode provided by the present invention also includes a deformation structure three. Specifically, the insulating substrate 1-1 and the substrate electrode 1-2 together constitute 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 FIG7 . The structure has three electrode layers: a substrate electrode 1-2, a bottom electrode 5-2 and a top electrode 7. On the one hand, the structure can realize electrostatic shielding by grounding the substrate electrode 1-2 and the top electrode 7, thereby improving electrical safety. On the other hand, the substrate electrode 1-2 can be used as an electrostatic electrode, so that the flexible bottom electrode 5 can be deformed more flexibly.

The micromechanical ultrasonic transducer with a central support bottom electrode provided by the present invention also includes a deformation structure 4. Specifically, the bendable 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. Through the piezoelectric effect, the shape of the bendable bottom electrode 5 can be adjusted, as shown in FIG8 .

The micromechanical ultrasonic transducer with a central support bottom electrode provided by the present invention also includes a deformation structure 5. 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. Through the piezoelectric effect, the shape of the vibration membrane 6 can be adjusted, as shown in FIG9 .

The present invention also provides a method for processing the micromechanical ultrasonic transducer, including a sacrificial layer release method, as shown in FIG10 , and the specific steps are:

Step 1, prepare a substrate wafer;

Step 2, depositing a first sacrificial layer on the substrate wafer, and performing patterning etching;

Step 3, depositing a conductively doped central support and bottom electrode layer;

Step 4, depositing a second sacrificial layer and performing patterning etching;

Step 5: deposit edge support and vibration membrane;

Step 6, releasing the first sacrificial layer and the second sacrificial layer by means of an etching solution to form a cavity;

Step 7: deposit a top electrode and perform patterning etching; finally, a micromechanical ultrasonic transducer unit with a central supporting bottom electrode is obtained.

The processing method of the micromechanical ultrasonic transducer provided by the present invention further includes a wafer bonding method, as shown in FIG11 , and the specific steps are as follows:

Step 1, preparing a first substrate wafer;

Step 2, etching a cavity on the first substrate wafer;

Step 3, prepare a polished second SOI wafer, including a substrate silicon layer, a buried oxide layer and a device layer; bond the first wafer in step 2 to the second SOI wafer;

Step 4, removing the substrate silicon layer and buried oxide layer of the second SOI wafer;

Step 5, etching the device layer to expose the cavity;

Step 6, prepare a third SOI wafer with a thick device layer, including a substrate silicon layer, a buried oxide layer and a device layer;

Step 7, patterning and etching the third SOI wafer;

Step 8, bonding the third SOI wafer to the wafer obtained in step 5;

Step 9, removing the substrate silicon layer and buried oxide layer of the third SOI wafer;

Step 10: deposit a top electrode and perform patterning etching; finally, a micromechanical ultrasonic transducer unit with a central supporting bottom electrode is obtained.

Fig. 3 shows a schematic diagram of the deformation of the micromechanical ultrasonic transducer under a DC bias voltage. The vibrating membrane 6 is bounded by the edge support 3, the edge is fixed, and the center is mainly deformed. The flexible bottom electrode 5 is anchored by the center support 2, the center is fixed, and the edge is mainly deformed.

Fig. 4 shows a schematic diagram of deformation of a conventional micromechanical ultrasonic transducer under a DC bias voltage. The vibrating membrane 6 is bounded by the edge support 3, the edge is fixed, and the center deforms mainly. The bottom electrode 5 is fixed on the substrate 1 and does not deform.

Figures 12 and 13 show the simulation results of the static deformation of the vibration layer and bottom electrode of the micromechanical ultrasonic transducer with the central column supporting the bottom electrode and the traditional structure micromechanical ultrasonic transducer, respectively. By comparison, it can be seen that the vibration membrane of the micromechanical transducer proposed in the present invention has a larger static deformation.

Figures 14, 15 and 16 respectively show the comparison of the average displacement of the vibrating membrane, the change of fractional capacitance and the resonant frequency of the micromechanical ultrasonic transducer with the central column supporting the bottom electrode and the micromechanical ultrasonic transducer with the traditional structure. It can be seen that with the increase of the DC bias voltage, the micromechanical ultrasonic transducer shows more obvious average displacement of the vibrating membrane, change of fractional capacitance and change of resonant frequency.

Figures 17 and 18 show the comparison of the average vibration amplitude and far-field sound pressure of the micromechanical ultrasonic transducer with the central column supporting the bottom electrode and the micromechanical ultrasonic transducer with the traditional structure. It can be seen that the average vibration amplitude peak value and far-field sound pressure peak value of the micromechanical ultrasonic transducer are much greater than those of the traditional structure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of the basic structure of a MUT in which a central column supports a bottom electrode according to the present invention.

FIG. 2 is a schematic diagram of the basic structure of a MUT with a center wall supporting a bottom electrode according to the present invention.

FIG. 3 is a schematic diagram of static deformation of a MUT with a center-supported bottom electrode according to the present invention.

FIG. 4 is a schematic diagram of static deformation of a conventional structure MUT.

FIG. 5 is a first deformation structure of the MUT with a centrally supported bottom electrode according to the present invention.

FIG. 6 is a second deformation structure of the MUT with a centrally supported bottom electrode according to the present invention.

FIG. 7 is a third deformation structure of the MUT with a centrally supported bottom electrode according to the present invention.

FIG. 8 is a fourth deformation structure of the MUT with a central supporting bottom electrode according to the present invention.

FIG. 9 is a fifth deformation structure of the MUT with a centrally supported bottom electrode according to the present invention.

FIG. 10 is a flow chart of the sacrificial layer release process of the MUT with a center-supported bottom electrode according to the present invention.

FIG. 11 is a flow chart of a wafer bonding process of a MUT with a center-supported bottom electrode according to the present invention.

FIG. 12 is a simulation result of static deformation of the vibration layer and the bottom electrode of the MUT in which the center column supports the bottom electrode according to the present invention.

FIG. 13 is a simulation result of static deformation of the vibration layer and bottom electrode of a conventional structure MUT.

FIG. 14 is a simulation result comparing the relationship between the average displacement of the vibrating membrane of the MUT with the bottom electrode supported by the central column of the present invention and the MUT with the traditional structure and the DC bias voltage.

FIG. 15 is a simulation result comparing the relationship between the change in fractional capacitance of the MUT with the bottom electrode supported by the central pillar of the present invention and the MUT with the traditional structure and the change in the DC bias voltage.

FIG. 16 is a simulation result comparing the relationship between the resonant frequency of the MUT with the central pillar supporting the bottom electrode of the present invention and the MUT with the traditional structure and the DC bias voltage.

FIG. 17 is a simulation result comparing the average vibration amplitude of the vibration membrane of the MUT with the bottom electrode supported by the central column of the present invention and the MUT with a traditional structure.

FIG. 18 is a simulation result comparing the far-field emission sound pressure of the MUT with the bottom electrode supported by the central pillar of the present invention and the MUT with a traditional structure.

DETAILED DESCRIPTION

The present invention will be described in more detail below with reference to the accompanying drawings. In each of the accompanying drawings, the same elements are represented by similar reference numerals. For the sake of clarity, the various parts in the accompanying drawings are not drawn to scale. In addition, some well-known parts may not be shown.

Many specific details of the present invention are described below, such as device structure, material size processing technology and technology, so as to make the present invention more clearly understood. However, as can be understood by those skilled in the art, the present invention can be implemented without following these specific details.

FIG. 1 is a schematic diagram showing the basic structure of a MUT in which a central column supports a bottom electrode according to the present invention.

Referring to the cross-section and top view in the figure, the MUT with a central pillar supporting a bottom electrode proposed in the present invention includes, arranged from bottom to top: a substrate 1, a central support 2, an edge support 3, a cavity 4, a flexible bottom electrode 5, a vibrating membrane 6, and a top electrode 7. The MUT transducer unit with a central pillar supporting a bottom electrode is circular as a whole, and its components, the substrate 1, the central support 2, the flexible bottom electrode 5, the vibrating membrane 6, and the top electrode 7, are all circular.

The center support 2 and the edge support 3 are fixed on the substrate, supporting the flexible bottom electrode 5 and the vibration membrane 6 respectively; the cavity 4 is surrounded by the edge support 3 and the vibration membrane 6, providing space for the flexible bottom electrode 5 and the vibration membrane 6 to move up and down; the flexible bottom electrode 5 is located inside the cavity 4; the top electrode 7 is located above the vibration membrane 6, and its area is usually less than or equal to the vibration membrane 6. The deformation of the top electrode 7 and the vibration membrane 6 is basically synchronized.

The thickness of the substrate 1 is 500μm, and its material is borosilicate glass. The central support 2 and the flexible bottom electrode 5 are usually made of conductive doped silicon. The height of the central support 2 is 1μm and the radius is 2.5μm. The thickness of the flexible bottom electrode 5 is 2μm and the radius is 14μm. The materials of the edge support 3 and the vibration membrane 6 are silicon. The edge support 3 is 15μm, and its top is the vibration membrane 6, and the gap height with the flexible bottom electrode 5 is 100nm. The thickness of the vibration membrane 6 is 1.5μm. The material of the top electrode 7 is gold, and its radius is 10μm and the thickness is 300nm. The flexible bottom electrode 5 is anchored to the substrate 1 by the central support 2, and its edge can be deformed to a large extent. The vibration membrane 6 is bounded by the edge support 3, and its deformation is mainly in the center.

FIG. 2 is a schematic diagram showing the basic structure of a MUT with a central wall supporting a bottom electrode according to the present invention.

Referring to the cross-section and top view in the figure, the MUT with a central wall supporting a bottom electrode proposed in the present invention includes, arranged from bottom to top: a substrate 1, a central support 2, an edge support 3, a cavity 4, a flexible bottom electrode 5, a vibrating membrane 6, and a top electrode 7. The MUT transducer unit with a central wall supporting a bottom electrode is rectangular as a whole, and its components, the substrate 1, the central support 2, the flexible bottom electrode 5, the vibrating membrane 6, and the top electrode 7, are all rectangular.

The center support 2 and the edge support 3 are fixed on the substrate, supporting the flexible bottom electrode 5 and the vibration membrane 6 respectively; the cavity 4 is surrounded by the edge support 3 and the vibration membrane 6, providing space for the flexible bottom electrode 5 and the vibration membrane 6 to move up and down; the flexible bottom electrode 5 is located inside the cavity 4; the top electrode 7 is located above the vibration membrane 6, and its area is usually less than or equal to the vibration membrane 6. The deformation of the top electrode 7 and the vibration membrane 6 is basically synchronized.

The thickness of the substrate 1 is 500μm, and its material is borosilicate glass. The central support 2 and the flexible bottom electrode 5 are usually made of conductive doped silicon. The height of the central support 2 is 1μm and the radius is 2.5μm. The thickness of the flexible bottom electrode 5 is 2μm and the radius is 14μm. The materials of the edge support 3 and the vibration membrane 6 are silicon. The edge support 3 is 15μm, and its top is the vibration membrane 6, and the gap height with the flexible bottom electrode 5 is 100nm. The thickness of the vibration membrane 6 is 1.5μm. The material of the top electrode 7 is gold, and its radius is 10μm and the thickness is 300nm. The flexible bottom electrode 5 is anchored to the substrate 1 by the central support 2, and its edge can be deformed to a large extent. The vibration membrane 6 is bounded by the edge support 3, and its deformation is mainly in the center.

FIG. 3 is a schematic diagram showing the static deformation of the MUT with a centrally supported bottom electrode according to the present invention.

As shown in FIG3 , for the micromechanical transducer of the present invention, the vibrating membrane 6 is bounded by the edge support 3, the edge is fixed, and the center is mainly deformed. The flexible bottom electrode 5 is anchored by the center support 2, the center is fixed, and the edge is mainly deformed. When a DC bias voltage is applied between the top electrode 7 and the flexible bottom electrode 5, the center of the vibrating membrane 6 is concave downward, and the edge of the flexible bottom electrode 5 is warped upward, and the distance between them is reduced.

FIG. 4 is a schematic diagram showing static deformation of a conventional structure MUT.

The vibration membrane 6 is bounded by the edge support 3, the edge is fixed, and the center deformation is mainly. When a DC bias voltage is applied between the top electrode 7 and the bottom electrode 5, the center of the vibration membrane 6 is sunken, the bottom electrode 5 is not deformed, and the distance between them is reduced.

FIG. 5 shows a first deformation structure of the MUT with a centrally supported bottom electrode according to the present invention.

As shown in Fig. 5, the characteristic of the first deformed structure is that the constituent materials of the substrate 1, the central support 2 and the bendable bottom electrode 5 are all conductively doped silicon. This structure is generally processed by a wafer bonding process.

FIG. 6 shows a second deformation structure of the MUT with a centrally supported bottom electrode according to the present invention.

As shown in FIG6 , the second deformation structure is characterized in that the central support 2 is a circular ring column with a hollow center. The thickness of the ring is 3 μm and the outer diameter is 8 μm, which is smaller than the radius of the bendable bottom electrode 5, which is 14 μm. The central support 2 can also be two or more rows of parallel rectangular walls with central symmetry. This annular support structure has better mechanical robustness.

FIG. 7 shows a third deformation structure of the MUT with a centrally supported bottom electrode according to the present invention.

As shown in FIG7 , the deformation structure three is characterized in that the insulating substrate 1-1 and the 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. The structure has three electrode layers: the substrate electrode 1-2, the bottom electrode 5-2 and the top electrode 7. On the one hand, the structure can achieve electrostatic shielding by grounding the substrate electrode 1-2 and the top electrode 7, thereby improving electrical safety. On the other hand, the substrate electrode 1-2 can be used as an electrostatic electrode, so that the flexible bottom electrode 5 can be deformed more flexibly.

FIG. 8 shows a fourth deformation structure of the MUT with a centrally supported bottom electrode according to the present invention.

As shown in Fig. 8, the deformation structure 4 is characterized in that the bendable 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 shape of the bendable bottom electrode 5 can be adjusted through the piezoelectric effect.

FIG. 9 shows a fifth deformation structure of the MUT with a centrally supported bottom electrode according to the present invention.

As shown in Fig. 9, the deformation structure 5 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 shape of the vibration membrane 6 can be adjusted through the piezoelectric effect.

FIG. 10 is a flowchart showing a sacrificial layer release process of a MUT with a centrally supported bottom electrode according to the present invention.

As shown in FIG. 10 , the sacrificial layer release process flow of the MUT with a center-supported bottom electrode includes the following steps:

Step 1: prepare a first substrate wafer, the material of which is borosilicate glass;

Step 2: deposit a first sacrificial layer on the first substrate wafer, and perform patterning etching to define the pattern of the cavity.

Step 3, depositing a conductively doped central support and bottom electrode layer, the material of which is silicon;

Step 4, depositing a second sacrificial layer, and performing patterning etching on its edge;

Step 5: deposit edge support and vibration membrane, the material is silicon;

Step 6, corroding the first sacrificial layer and the second sacrificial layer by injecting a specific corrosive liquid into the etching hole to form a cavity;

Step 7: Deposit gold and perform patterning etching to form a top electrode. Finally, a MUT unit with a center supporting bottom electrode is obtained.

FIG. 11 is a flowchart showing a wafer bonding process of a MUT with a center-supported bottom electrode according to the present invention.

As shown in FIG. 11 , the wafer bonding process flow of the MUT with a center-supported bottom electrode includes the following steps:

Step 1: prepare a first substrate wafer, the material of which is silicon;

Step 2: performing pattern etching on the first substrate wafer to define a cavity;

Step 3, prepare a polished second SOI wafer, including a substrate silicon layer, a buried oxide layer and a device layer; bond the first wafer in step 2 to the second SOI wafer;

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, prepare a third SOI wafer with a thick device layer, including a substrate silicon layer, a buried oxide layer and a device layer;

Step 7, pattern-etching the device layer of the third SOI wafer, and using the layer with a thickness of 1.5 μm as a vibration membrane;

Step 8, bonding the third SOI wafer to the wafer obtained in step 5;

Step 9, removing the substrate silicon layer and buried oxide layer of the third SOI wafer 3;

Step 10: deposit gold and perform patterning etching to form a top electrode; finally, a MUT unit with a center supporting bottom electrode is obtained.

FIG. 12 shows the simulation results of static deformation of the vibration layer and the bottom electrode of the MUT in which the bottom electrode is supported by the central column of the present invention.

As shown in FIG12 , when different DC bias voltages are applied, the edge of the vibrating membrane (solid line in the figure) of the MUT with the center column supporting the bottom electrode of the present invention is fixed, and the center is sunken downward, while the center of the bendable bottom electrode (dashed line in the figure) is fixed, and the edge is raised. The distance between the two is reduced.

FIG. 13 shows the static deformation simulation results of the vibration layer and bottom electrode of the conventional structure MUT.

As shown in Figure 13, when different DC bias voltages are applied, the vibrating membrane (solid line in the figure) of the conventional MUT has a fixed edge and a concave center, while the bottom electrode (dashed line in the figure) is fixed and does not deform. The distance between the two decreases.

FIG. 14 shows a comparison of simulation results of the relationship between the average displacement of the vibrating membrane of the MUT with the bottom electrode supported by the central column of the present invention and the MUT with a traditional structure and the DC bias voltage.

As shown in Figure 14, compared with the conventional structure MUT, the MUT with the center column supporting the bottom electrode of the present invention exhibits a larger vibration membrane deformation, and the difference becomes more obvious with the increase of the DC bias voltage. This is because the electrostatic force caused by the upward warping of the flexible bottom electrode is enhanced.

FIG. 15 shows a comparison of simulation results of the relationship between the change in fractional capacitance of the MUT with the center pillar supporting the bottom electrode of the present invention and the MUT with a traditional structure and the change in the DC bias voltage.

For the static capacitance value of the MUT, the fractional capacitance change is defined as the percentage of the capacitance under the DC bias voltage divided by the capacitance without the DC bias voltage. As shown in FIG15 , the MUT of the present invention with the center column supporting the bottom electrode exhibits a larger fractional capacitance change, and this difference becomes more obvious as the DC bias voltage increases. This is mainly due to the upward warping of the flexible bottom electrode, which further reduces the distance between the electrodes.

FIG. 16 shows a comparison of simulation results of the relationship between the resonant frequency of the MUT with the center column supporting the bottom electrode of the present invention and the MUT with a traditional structure and the DC bias voltage.

Due to the nonlinear effect of electrostatic force, CMUT exhibits a spring softening effect, that is, the resonant frequency decreases with the increase of DC bias voltage. As shown in FIG16 , compared with the conventional structure MUT, the resonant frequency of the MUT with the center column supporting the bottom electrode of the present invention has a more obvious shift. This shows that its spring softening effect is more obvious.

FIG. 17 shows a comparison of simulation results of the average vibration amplitude of the vibration membrane of the MUT with the bottom electrode supported by the central pillar of the present invention and the MUT with a traditional structure.

As shown in Figure 17, the frequency characteristics of the average amplitude of the vibrating membrane of the MUT with the central column supporting the bottom electrode of the present invention and the MUT with the traditional structure under 10-20MHz AC small signal excitation are simulated and analyzed. Compared with the MUT with the traditional structure, the average amplitude peak of the MUT with the central column supporting the bottom electrode of the present invention is about 240nm, which is much larger 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 with a central pillar supporting a bottom electrode according to the present invention and the MUT with a traditional structure.

As shown in Figure 18, the frequency characteristics of the far-field emission sound pressure of the MUT with the central column supporting the bottom electrode of the present invention and the MUT with the traditional structure under 10-20MHz AC small signal excitation 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 of the present invention at a distance of 3mm is about 1100Pa, which is much greater than the 280Pa of the traditional structure. This shows that the MUT with the central column supporting the bottom electrode of the present invention has a higher emission sound pressure. .

Without departing from the spirit and scope of the present invention, any ordinary technician in the field can make many variations and modifications based on the contents disclosed by the present invention, which should also be considered as the protection scope of the present 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.