An isoelectric ring capacitive micromachined ultrasonic transducer and a method of manufacturing the same

Abstract

The application discloses an equal-potential ring capacitive micromachined ultrasonic transducer and a preparation method thereof, and belongs to the technical field of micro-electro-mechanical systems, which comprises: a residual substrate as a lower electrode; a lower insulating layer; a support for defining a cavity; a vibratable diaphragm covering above the support, which forms the cavity together with the support and the lower insulating layer; an upper insulating layer; an upper electrode formed above the upper insulating layer and projected in the cavity region; and an equal-potential ring formed above the upper insulating layer and surrounding the upper electrode, the projection of the equal-potential ring is in the support region, and the equal-potential ring is electrically insulated from the upper electrode. The application increases the equal-potential ring at the support, makes the electric field distribution more uniform and stable, reduces the edge effect and parasitic capacitance of the support region, and thus realizes a high electromechanical coupling coefficient.

Description

Technical Field

[0001] This invention belongs to the field of microelectromechanical systems (MEMS) technology, specifically relating to an equipotential ring capacitor type micromechanical ultrasonic transducer and its preparation method. Background Technology

[0002] Ultrasound, due to its good directionality, concentrated energy, and strong penetrating power, has been widely used in non-destructive testing, medical imaging and treatment, and biochemical detection. As the core component of an ultrasonic technology system, the performance of the ultrasonic transducer directly affects the application effect of ultrasonic technology. Traditional piezoelectric ceramic ultrasonic transducers (PZTs) have limitations such as large size, narrow bandwidth, and difficulty in integration, which restrict the further development of ultrasonic technology in some fields.

[0003] Capacitive micromachined ultrasonic transducers (CMUTs) are a novel type of ultrasonic transducer developed based on microelectromechanical systems (MEMS) technology. A CMUT mainly consists of a deformable thin film and a substrate, separated by pillars, cavities, and insulating layers. During transmission, applying a DC bias voltage and an AC excitation voltage causes the thin film to vibrate, generating ultrasonic waves. During reception, external sound waves cause the thin film to vibrate, changing its capacitance and generating an electrical signal. Compared to traditional piezoelectric ceramic ultrasonic transducers, CMUTs offer advantages such as miniaturization, high bandwidth, high electromechanical coupling coefficient, and ease of integration with integrated circuits. They exhibit significant advantages in ultrasonic imaging, biometric fingerprinting, non-contact operation and control, making them a potential candidate for next-generation mainstream ultrasonic transducers.

[0004] However, to improve acoustic emission power, CMUTs are typically operated in arrays consisting of tens to thousands of units connected in parallel. Therefore, the capacitance caused by the electrode connections between units, the support area covered by electrode pads, and the edge effects of the upper and lower electrodes of the CMUT units are the main sources of parasitic capacitance. The larger the parasitic capacitance of the CMUT, the lower the electromechanical conversion efficiency, the smaller the film vibration amplitude, and the worse its performance, while also increasing the difficulty of signal detection. Therefore, how to reduce parasitic capacitance and improve the electromechanical coupling coefficient is a key issue in CMUT research. Summary of the Invention

[0005] The purpose of this invention is to provide an equipotential ring capacitive micromechanical ultrasonic transducer and its fabrication method. By adding an equipotential ring to the support column to reduce parasitic capacitance, the electromechanical coupling coefficient is improved, and the fabrication process is relatively simple to overcome the shortcomings of the prior art.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows: In a first aspect, the present invention provides an equipotential ring capacitive micromechanical ultrasonic transducer, comprising a substrate, wherein the substrate serves as a lower electrode; A lower insulating layer is formed on the upper surface of the substrate; At least one pillar is formed above the lower insulating layer to define a region of at least one cavity; A vibrating diaphragm covers the top of the support column and together with the support column and the lower insulating layer, forms the cavity; An upper insulating layer covers the upper surface of the film; At least one upper electrode is formed above the upper insulating layer, and the projection of the upper electrode in the vertical direction is located within the region of the cavity; At least one equipotential ring is formed above the upper insulating layer and surrounds the upper electrode; The projection of the equipotential ring in the vertical direction is located within the area of ​​the support column, and the equipotential ring and the upper electrode are spaced apart from each other and electrically insulated.

[0007] Furthermore, the horizontal width of the equipotential ring is smaller than the horizontal width of the support column, and the projection of the equipotential ring in the vertical direction falls completely within the area of ​​the support column.

[0008] Furthermore, the horizontal distance between the equipotential ring and the upper electrode is 0.1 μm to 10 μm, and the thickness of the equipotential ring and the upper electrode is the same, both being 0.1 μm to 2 μm.

[0009] Furthermore, the radius of the upper electrode is 10μm to 200μm, and the thickness of the substrate is 1μm to 20μm.

[0010] Furthermore, the thickness of the upper insulating layer is 0.1 μm to 2 μm, and the thickness of the lower insulating layer is 0.1 μm to 2 μm.

[0011] Furthermore, the width of the equipotential ring is 1 μm to 40 μm, and the thickness of the film is 0.1 μm to 5 μm.

[0012] Furthermore, the width of the support column is 1μm to 40μm, and the height is 0.1μm to 10μm.

[0013] Furthermore, the width of the cavity is 10μm to 200μm, and the height is 0.1μm to 10μm.

[0014] Secondly, the present invention provides a capacitive micromechanical ultrasonic transducer array, comprising multiple equipotential ring capacitive micromechanical ultrasonic transducers arranged in parallel.

[0015] Thirdly, the present invention provides a method for fabricating a capacitive micromechanical ultrasonic transducer unit, comprising the following steps: Step 1: Provide a substrate; Step 2: Form a first silicon dioxide layer and a second silicon dioxide layer on the upper and lower surfaces of the substrate, respectively; Step 3: After photolithographically patterning the first silicon dioxide layer to obtain the cavity region, dry etching technology is used to form the cavity and pillars, while the part located at the bottom of the cavity is retained as the lower insulating layer to obtain a substrate with a cavity. Step 4: Provide an SOI wafer, the SOI wafer comprising a substrate silicon layer, a buried oxide layer and a device silicon layer stacked sequentially; Step 5: Bond the bonding surface of the silicon layer of the device to the upper surface of the cavity substrate; Step 6: Reduce the thickness of the substrate silicon layer; Step 7: Remove the remaining substrate silicon layer and buried oxide layer to expose the device silicon layer and form a vibrating thin film; Step 8: Form an upper insulating layer on the upper surface of the film; Step 9: On the upper insulating layer, and corresponding to the position of the pillar, sputter metal and pattern it to form an equipotential ring; on the upper insulating layer, and corresponding to the position of the cavity, sputter metal and pattern it to form an upper electrode, the upper electrode being spaced apart from and electrically insulated from the equipotential ring; Step 10: Remove the second silicon dioxide layer on the lower surface of the substrate and use the exposed substrate directly as the lower electrode.

[0016] Compared with the prior art, the present invention has the following beneficial technical effects: This invention provides an equipotential ring capacitive micromechanical ultrasonic transducer (CMUT). An equipotential ring is added above a support column. This equipotential ring is electrically insulated from the upper electrode and located within the projection area of ​​the support column. During operation, the same DC bias voltage is applied to the upper electrode and the equipotential ring. The electric field between the equipotential ring and the lower electrode effectively suppresses the edge electric field effect of the upper and lower electrodes in the support column region, thereby significantly reducing the parasitic capacitance generated in this region. The reduction in parasitic capacitance directly improves the electromechanical conversion efficiency of the CMUT, enabling the thin film to achieve a larger vibration amplitude under the same excitation, thus achieving a higher electromechanical coupling coefficient.

[0017] This invention provides a method for fabricating an equipotential ring capacitive micromechanical ultrasonic transducer. Compared with other methods for reducing parasitic capacitance, such as complex electrode structure design and additional shielding layers, this invention only requires adding an equipotential ring above the support pillar. This equipotential ring can be completed in the same metal sputtering and patterning process as the upper electrode, without the need for additional photolithography or process steps. The fabrication process is relatively simple, highly compatible with existing MEMS processes, and easy to industrialize. Attached Figure Description

[0018] Figure 1 This is a schematic diagram of an equipotential ring capacitor type micromechanical ultrasonic transducer in an embodiment of the present invention.

[0019] Figure 2 This is a schematic diagram of a capacitive micromechanical ultrasonic transducer array structure in an embodiment of the present invention.

[0020] Figure 3 This is a schematic diagram illustrating the fabrication process of an equipotential ring capacitive micromechanical ultrasonic transducer according to an embodiment of the present invention.

[0021] In the figure, 1 is the upper electrode; 2 is the equipotential ring; 3 is the upper insulating layer; 4 is the thin film; 5 is the pillar; 6 is the cavity; 7 is the lower insulating layer; 8 is the substrate; 9 is the upper electrode pad; 10 is the equipotential ring pad; 41 is the device silicon layer; 42 is the buried oxide layer; 43 is the substrate silicon layer; 81 is the substrate; 82 is the first silicon dioxide layer; and 83 is the second silicon dioxide layer. Detailed Implementation

[0022] In the following description, only certain exemplary embodiments are briefly described. As those skilled in the art will recognize, the described embodiments can be modified in various ways without departing from the spirit or scope of the invention. Therefore, the drawings and description are considered to be exemplary in nature and not restrictive.

[0023] In the description of this invention, it should be understood that the terms "center," "longitudinal," "lateral," "length," "width," "thickness," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," "clockwise," "counterclockwise," "axial," "radial," and "circumferential" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are used only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention.

[0024] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of this invention, "a plurality of" means two or more, unless otherwise explicitly specified.

[0025] In this invention, unless otherwise explicitly specified and limited, the terms "installation," "connection," "linking," and "fixing," etc., should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a mechanical connection, an electrical connection, or a communication connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0026] In this invention, unless otherwise explicitly specified and limited, "above" or "below" the second feature can include direct contact between the first and second features, or contact between the first and second features through another feature between them. Furthermore, "above," "over," and "on top" of the second feature includes the first feature being directly above or diagonally above the second feature, or simply indicates that the first feature is at a higher horizontal level than the second feature. "Below," "below," and "under" the second feature includes the first feature being directly above or diagonally above the second feature, or simply indicates that the first feature is at a lower horizontal level than the second feature.

[0027] It should be understood that, when used in this specification and the appended claims, the terms "comprising" and "including" indicate the presence of the described features, integrals, steps, operations, elements and / or components, but do not exclude the presence or addition of one or more other features, integrals, steps, operations, elements, components and / or collections thereof.

[0028] It should also be understood that the terminology used in this specification is for the purpose of describing particular embodiments only and is not intended to limit the invention. As used in this specification and the appended claims, the singular forms “a,” “an,” and “the” are intended to include the plural forms unless the context clearly indicates otherwise.

[0029] It should also be further understood that the term "and / or" as used in this specification and the appended claims refers to any combination of one or more of the associated listed items and all possible combinations, and includes such combinations.

[0030] The accompanying drawings illustrate various structural schematic diagrams according to embodiments disclosed in this invention. These drawings are not to scale, and some details have been enlarged for clarity, and some details may have been omitted. The shapes of the various regions and layers shown in the drawings, as well as their relative sizes and positional relationships, are merely exemplary and may deviate from reality due to manufacturing tolerances or technical limitations. Furthermore, those skilled in the art can design regions / layers with different shapes, sizes, and relative positions as needed.

[0031] See Figure 1 This invention provides an equipotential ring capacitive micromechanical ultrasonic transducer, comprising an upper electrode 1, an equipotential ring 2, an upper insulating layer 3, a thin film 4, a support 5, a cavity 6, a lower insulating layer 7, and a substrate 8; the substrate 8 serves as the lower electrode. In this invention, the substrate 8 is made of low-resistivity single-crystal silicon material, whose low resistance characteristics ensure its suitability as a good conductive electrode. Preferably, the thickness of the substrate 8 ranges from 1 μm to 800 μm. When the substrate 8 thickness is 1 μm, the overall thickness of the capacitive micromechanical ultrasonic transducer unit is relatively small, suitable for applications with extremely stringent size requirements; when the substrate 8 thickness is 800 μm, the substrate 8 has higher mechanical strength and can provide more stable support. By controlling the substrate 8 thickness within this range, a balance can be achieved between miniaturization and structural stability of the capacitive micromechanical ultrasonic transducer unit.

[0032] The lower insulating layer 7 is formed on the upper surface of the substrate 8, located at the bottom of the cavity 6 and below the pillar 5. The main function of the lower insulating layer 7 is to achieve electrical insulation between the thin film 4 and the substrate 8, preventing direct conduction between the upper and lower electrodes. At the same time, as the bottom interface of the cavity 6, the lower insulating layer 7 plays an important role in the precise control of the height of the cavity 6. In a more specific embodiment of the present invention, the thickness of the lower insulating layer 7 ranges from 0.1 μm to 2 μm. When the thickness of the lower insulating layer 7 is 0.1 μm, the lower insulating layer 7 is relatively thin, which is beneficial for reducing the overall thickness of the capacitive micromechanical ultrasonic transducer unit, while also resulting in less electric field attenuation. When the thickness of the lower insulating layer 7 is 2 μm, the insulation performance is better, enabling it to withstand higher operating voltages and preventing breakdown. By controlling the thickness of the lower insulating layer 7 within this range, a balance can be achieved between insulation performance and electric field strength.

[0033] At least one support post 5 is formed above the lower insulating layer 7 to support the thin film 4 and define the area of ​​the cavity 6. The support post 5 is a fixed support point for the thin film 4, and its structural stability directly affects the reliability and consistency of the vibration of the thin film 4. At the same time, the area of ​​the support post 5 is also the main source of edge effects and parasitic capacitance in traditional CMUTs. The equipotential ring 2 of the present invention is specifically optimized for the area of ​​the support post 5. In a more specific embodiment of the present invention, the width of the support column 5 ranges from 1 μm to 40 μm, and the height ranges from 0.1 μm to 10 μm. When the width of the support column 5 is 1 μm, the area occupied by the support column 5 is small, which is beneficial to improving the fill factor and array density of the cells; when the width of the support column 5 is 40 μm, the support column 5 has a larger support area, which can provide more stable mechanical support. When the height of the support column 5 is 0.1 μm, the height of the cavity 6 is small, which is beneficial to improving the electric field strength and sensitivity; when the height of the support column 5 is 10 μm, the height of the cavity 6 is large, and the vibration space of the thin film 4 is more sufficient, which is beneficial to large amplitude vibration. By controlling the size of the support column 5 within this range, a balance can be achieved between support stability, cell density, and vibration performance.

[0034] A vibrating diaphragm 4 covers the top of the support column 5 and together with the support column 5 and the lower insulating layer 7, forms a cavity 6; the cavity 6 provides space for the vibration of the diaphragm 4.

[0035] In a more specific embodiment of the present invention, the width of cavity 6 ranges from 10 μm to 200 μm, and the height ranges from 0.1 μm to 10 μm. When the width of cavity 6 is 10 μm, the size of the capacitive micromechanical ultrasonic transducer unit is small, which is beneficial for high-density array integration and suitable for high-resolution imaging. When the width of cavity 6 is 200 μm, the size of the capacitive micromechanical ultrasonic transducer unit is large, and the vibrating mass of the thin film 4 is large, which is suitable for low-frequency, high-power output. When the height of cavity 6 is 0.1 μm, the electric field strength is high, which is beneficial for improving sensitivity. When the height of cavity 6 is 10 μm, the vibration space of the thin film 4 is larger, and a larger vibration amplitude can be achieved. By controlling the size of cavity 6 within this range, the operating frequency and output power of the device can be adjusted according to different application requirements.

[0036] Thin film 4 is a vibrating silicon thin film and is the core vibrating element of the CMUT. In the transmitting mode, thin film 4 vibrates under the action of electrostatic force to generate ultrasonic waves; in the receiving mode, external ultrasonic waves cause thin film 4 to vibrate, changing the capacitance value and thus generating an electrical signal.

[0037] In a more specific embodiment of the present invention, the thin film 4 is made of low-resistivity single-crystal silicon material with a resistivity of less than 0.001 Ω·cm, ensuring good conductivity and enabling a good electrical connection with the upper electrode 1. The thickness of the thin film 4 ranges from 0.1 μm to 5 μm. When the thickness of the thin film 4 is 0.1 μm, it is very thin, has low stiffness, is prone to vibration, and has high sensitivity, making it suitable for low-frequency or high-sensitivity applications. When the thickness of the thin film 4 is 5 μm, it is thicker, has greater stiffness, and can withstand higher driving voltages and larger vibration amplitudes, making it suitable for high acoustic power output. By controlling the thickness of the thin film 4 within this range, the frequency characteristics and output performance of the device can be adjusted according to application requirements.

[0038] An upper insulating layer 3 covers the upper surface of the thin film 4. The upper insulating layer 3 serves to achieve electrical insulation between the upper electrode 1, the equipotential ring 2, and the thin film 4. At the same time, the upper insulating layer 3 also protects the surface of the thin film 4, preventing short circuits or alloying caused by direct contact between the electrode metal and the silicon thin film.

[0039] In a more specific embodiment of the present invention, the thickness of the upper insulating layer 3 ranges from 0.1 μm to 2 μm. When the thickness of the upper insulating layer 3 is 0.1 μm, the insulating layer 3 is relatively thin, which is beneficial for reducing the distance between the upper electrode 1 and the thin film 4 and increasing the electric field strength. When the thickness of the upper insulating layer 3 is 2 μm, the insulation performance is better, and it can withstand higher operating voltages and prevent breakdown. By controlling the thickness of the upper insulating layer 3 within this range, a balance can be achieved between insulation reliability and electric field strength.

[0040] At least one upper electrode 1 is formed above the upper insulating layer 3, and the projection of the upper electrode 1 in the vertical direction is located within the region of the cavity 6. Electrode 1 is the main electrode for applying AC excitation voltage and DC bias voltage, and together with the lower electrode substrate 8, it forms the driving electric field and the detection capacitor.

[0041] In a more specific embodiment of the present invention, the radius of the upper electrode 1 ranges from 10 μm to 200 μm, and the thickness ranges from 0.1 μm to 2 μm. When the radius of the upper electrode 1 is 10 μm, the electrode area is small, which is suitable for high-density array integration; when the radius of the upper electrode 1 is 200 μm, the electrode area is large, which can generate greater electrostatic force, making it suitable for high acoustic power output. When the thickness of the upper electrode 1 is 0.1 μm, the electrode is thin and has a small mass, resulting in less impact on the added mass of the thin film vibration; when the thickness of the upper electrode 1 is 2 μm, the electrode is thicker, resulting in better conductivity and lower resistance. By controlling the size of the upper electrode 1 within this range, a balance can be achieved between electrode area, added mass, and conductivity.

[0042] At least one equipotential ring 2 is formed above the upper insulating layer 3 and surrounds the upper electrode 1; the projection of the equipotential ring 2 in the vertical direction is located within the region of the support column 5, and the equipotential ring 2 is spaced apart from the upper electrode 1 and electrically insulated from it. The main function of the equipotential ring 2 is to suppress the edge electric field effect in the region of the support column 5, reduce parasitic capacitance, and at the same time make the electric field distribution between the upper electrode 1 and the lower electrode more uniform and stable.

[0043] During operation, the same DC bias voltage is applied to the upper electrode 1 and the equipotential ring 2. The electrostatic field formed between the equipotential ring 2 and the lower electrode substrate 8 has the following two important functions: The electric field generated by the equipotential ring 2 makes the electric field distribution between the upper electrode 1 and the lower electrode more uniform and stable, reducing electric field divergence in the edge region. Therefore, under the same AC excitation voltage, the thin film 4 experiences a greater effective electrostatic force and stronger vibration amplitude, thereby improving the transmission power and receiving sensitivity. In traditional CMUTs, the pillar 5 region is the main area where edge effects of the upper and lower electrodes occur, resulting in large parasitic capacitances. The presence of the equipotential ring 2 "shields" the edge electric field of the upper electrode 1 in the pillar 5 region, causing the electric field lines in this region to be mainly concentrated between the lower electrode and the equipotential ring 2, rather than diverging directly from the edge of the upper electrode 1 to the lower electrode. This change significantly reduces the parasitic capacitance in the pillar 5 region, thereby improving the electromechanical coupling coefficient of the device. See Figure 2 The present invention provides a capacitive micromechanical ultrasonic transducer array, comprising multiple parallel-connected equipotential ring capacitive micromechanical ultrasonic transducers as described in Embodiment 1. The multiple units share the same substrate 8, and the upper electrode 1 and equipotential ring 2 of each unit are respectively connected to the upper electrode Pad9 and the equipotential ring Pad10 through independent leads.

[0044] In the array structure, each unit operates independently, but the equipotential ring 2 of all units can be connected to the same equipotential ring Pad10 and loaded with the same DC bias voltage.

[0045] In this embodiment, the array size is 30×30, totaling 900 units, with a unit spacing of 10μm. The upper electrode 1 of each unit has a radius of 50μm, the equipotential ring 2 has a width of 15μm, the support 5 has a width of 20μm, and the cavity 6 has a width of 100μm. The equipotential rings 2 of all units are connected to the equipotential ring Pad10 via a common bus and are subjected to the same DC bias voltage.

[0046] In transmission mode, the same DC bias voltage is applied to the upper electrode 1 and equipotential ring 2 of all units, while the same AC excitation voltage is applied to the upper electrode 1 of all units. The array as a whole generates a high-intensity ultrasonic beam, which is suitable for ultrasonic imaging or high-power therapeutic applications.

[0047] The working principle of this embodiment is as follows: In the transmission mode, the same DC bias voltage is applied to the upper electrode 1 and the equipotential ring 2 through the upper electrode Pad9 and the equipotential ring Pad10, respectively, while an AC excitation voltage is applied to the upper electrode 1 through the upper electrode Pad9. Since the equipotential ring 2 and the upper electrode 1 have the same DC potential, a stable electrostatic field is formed between the equipotential ring 2 and the lower electrode substrate 8. This electrostatic field effectively confines and shields the electric field lines of the upper electrode 1 and the lower electrode in the edge region of the support 5, making the electric field distribution below the upper electrode 1 more uniform and concentrated. Therefore, under the action of the AC excitation voltage, the effective electrostatic force on the thin film 4 is greater and more uniformly distributed, resulting in stronger ultrasonic output. Simultaneously, since the edge electric field in the support 5 region is effectively suppressed, the parasitic capacitance generated in this region is significantly reduced, improving the electromechanical conversion efficiency. Tests show that the electromechanical coupling coefficient of this embodiment is approximately 30% higher than that of a conventional CMUT without an equipotential ring.

[0048] See Figure 3 The present invention provides a method for fabricating a capacitive micromechanical ultrasonic transducer unit as described in Example 1, comprising the following specific steps: S1, a low-resistivity monocrystalline silicon wafer is selected as the substrate, and after standard RCA cleaning treatment, it serves as substrate 8. The resistivity of the low-resistivity monocrystalline silicon is less than 0.001 Ω·cm to ensure that substrate 8 can be used as a good lower electrode. In this embodiment, the initial thickness of substrate 8 is 500 μm, and this portion of the substrate will be directly used as the lower electrode; S2, a first silicon dioxide layer 82 and a second silicon dioxide layer 83 are formed on the upper and lower surfaces of the substrate 8 using a high-temperature oxidation process. In this embodiment, the oxidation temperature is 1050℃, the oxidation time is 2 hours, and the thickness of the formed silicon dioxide layer is 0.6μm. The first silicon dioxide layer 82 will subsequently form the lower insulating layer 7 and the pillar 5, and the second silicon dioxide layer 83 serves as the back protective layer. S3, the first silicon dioxide layer 82 is patterned using photolithography, coated with photoresist, and then exposed and developed to define the cavity region. Subsequently, the first silicon dioxide layer 82 is etched using reactive ion etching (RIE) technology to form the cavity 6 and the pillar 5. A mixture of CF4 and CHF3 is used as the etching gas, and the etching rate is approximately 0.1 μm / min. The etching depth is precisely controlled to 0.5 μm, leaving a 0.1 μm thick silicon dioxide layer at the bottom of the cavity 6 as the lower insulating layer 7. At this point, the resulting structure is a "substrate with a cavity," with raised pillars 5 and recessed cavities 6 on its surface, and the lower insulating layer 7 remaining at the bottom of the cavity 6. The width of the pillar 5 is 20 μm, and the width of the cavity 6 is 100 μm. S4, a low-resistivity and flat-surface SOI wafer is selected as the source of the thin film material. This SOI wafer comprises a substrate silicon layer 43 with a thickness of 400 μm, a buried oxide layer 42 with a thickness of 3 μm, and a device silicon layer 41 with a thickness of 2 μm, stacked sequentially. The resistivity of the device silicon layer 41 is less than 0.001 Ω·cm, and it will subsequently form the vibrating thin film 4. S5, using fused bonding technology, the bonding surface of the device silicon layer 41 of the SOI wafer is bonded to the upper surface of the cavity-containing substrate obtained in step 3. Before bonding, both surfaces are hydrophilically treated with a solution of NH4OH:H2O2:H2O=1:1:5 for 10 minutes, and then annealed at 1000℃ for 2 hours to enhance the bonding strength. After bonding, the device silicon layer 41 covers the pillar 5 and the cavity 6, and together with the pillar 5 and the lower insulating layer 7, forms a sealed cavity 6. S6. Chemical mechanical polishing (CMP) is used to reduce the thickness of the substrate silicon layer 43 of the SOI wafer. A silicon dioxide polishing slurry is used, with a polishing pressure of 4 psi and a polishing rate of approximately 2 μm / min. In this embodiment, approximately 75% of the thickness is reduced, thinning the substrate silicon layer 43 from 400 μm to 100 μm, leaving a thinner residual substrate silicon layer to reduce the burden on subsequent etching. S7, the remaining substrate silicon layer 43 is removed by deep reactive ion etching (DRIE), and the buried oxide layer 42 is removed by RIE etching, completely exposing the device silicon layer 41. At this point, the device silicon layer 41 becomes a vibrating thin film 4 with a thickness of 2 μm, covering the cavity 6 and the pillar 5. S8, a layer of silicon dioxide is deposited on the upper surface of the thin film 4 by plasma-enhanced chemical vapor deposition (PECVD) to form the upper insulating layer 3. The deposition temperature is 300°C, and in this embodiment, the thickness of the upper insulating layer 3 is 0.2 μm. S9. Photoresist is applied over the upper insulating layer 3, and the equipotential ring pattern above the pillar 5 is defined by photolithography. Then, gold is sputtered at a power of 200W for 5 minutes to form a gold layer with a thickness of 0.4μm. The photoresist and excess gold layer are removed by a lift-off process to form an equipotential ring 2 with a width of 15μm. Photoresist was applied again, and the upper electrode pattern above cavity 6 was defined using photolithography. Gold was sputtered at a power of 200W for 5 minutes to form a 0.4μm thick gold layer. The photoresist and excess gold layer were removed using a lift-off process to form upper electrode 1 with a radius of 50μm. During the patterning process, ensure that the upper electrode 1 and the equipotential ring 2 are spaced apart and electrically insulated from each other, with a horizontal spacing of 5 μm between them; S10, the second silicon dioxide layer 83 on the lower surface of the substrate 8 is removed by RIE etching, and the exposed substrate 81 is directly used as the lower electrode of the CMUT unit.

[0049] Thus, the fabrication of an equipotential ring capacitive micromechanical ultrasonic transducer was completed.

[0050] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely preferred examples and are not intended to limit the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the claimed invention.

Claims

1. An equipotential ring capacitive micromechanical ultrasonic transducer, characterized in that, Includes a substrate (8), which serves as the lower electrode; A lower insulating layer (7) is formed on the upper surface of the substrate (8); At least one support (5) is formed above the lower insulating layer (7) to define the area of ​​at least one cavity (6); A vibrating film (4) covers the top of the support (5) and together with the support (5) and the lower insulating layer (7) forms the cavity (6). An upper insulating layer (3) covers the upper surface of the thin film (4); At least one upper electrode (1) is formed above the upper insulating layer (3), and the projection of the upper electrode (1) in the vertical direction is located in the region of the cavity (6); At least one equipotential ring (2) is formed above the upper insulating layer (3) and is disposed around the upper electrode (1); The projection of the equipotential ring (2) in the vertical direction is located within the area of ​​the support (5), and the equipotential ring (2) and the upper electrode (1) are spaced apart from each other and electrically insulated.

2. The equipotential ring capacitive micromechanical ultrasonic transducer according to claim 1, characterized in that, The horizontal width of the equipotential ring (2) is smaller than the horizontal width of the support column (5), and the projection of the equipotential ring (2) in the vertical direction falls completely within the area of ​​the support column (5).

3. The equipotential ring capacitive micromechanical ultrasonic transducer according to claim 1, characterized in that, The horizontal distance between the equipotential ring (2) and the upper electrode (1) is 0.1μm to 10μm, and the thickness of the equipotential ring (2) and the upper electrode (1) is the same, both being 0.1μm to 2μm.

4. The equipotential ring capacitive micromechanical ultrasonic transducer according to claim 1, characterized in that, The radius of the upper electrode (1) is 10μm to 200μm, and the thickness of the substrate (8) is 1μm to 20μm.

5. The equipotential ring capacitive micromechanical ultrasonic transducer according to claim 1, characterized in that, The thickness of the upper insulating layer (3) is 0.1 μm to 2 μm, and the thickness of the lower insulating layer (7) is 0.1 μm to 2 μm.

6. The equipotential ring capacitive micromechanical ultrasonic transducer according to claim 1, characterized in that, The width of the equipotential ring (2) is 1 μm to 40 μm, and the thickness of the thin film (4) is 0.1 μm to 5 μm.

7. The equipotential ring capacitive micromechanical ultrasonic transducer according to claim 1, characterized in that, The width of the support column (5) is 1μm to 40μm and the height is 0.1μm to 10μm.

8. The equipotential ring capacitive micromechanical ultrasonic transducer according to claim 1, characterized in that, The cavity (6) has a width of 10μm to 200μm and a height of 0.1μm to 10μm.

9. A capacitive micromechanical ultrasonic transducer array, characterized in that, An equipotential ring capacitive micromechanical ultrasonic transducer according to any one of claims 1-8, comprising multiple transducers connected in parallel.

10. A method for fabricating an equipotential ring capacitive micromechanical ultrasonic transducer, comprising fabricating an equipotential ring capacitive micromechanical ultrasonic transducer as described in any one of claims 1-8, characterized in that, Includes the following steps: Step 1: Provide a substrate (8); Step 2: A first silicon dioxide layer (82) and a second silicon dioxide layer (83) are formed on the upper and lower surfaces of the substrate (8), respectively. Step 3: After photolithographically patterning the first silicon dioxide layer (82) to obtain the cavity region, the cavity (6) and pillar (5) are formed by dry etching technology, while the part located at the bottom of the cavity (6) is retained as the lower insulating layer (7) to obtain a substrate with a cavity. Step 4: Provide an SOI wafer, the SOI wafer comprising a substrate silicon layer (43), a buried oxide layer (42) and a device silicon layer (41) stacked sequentially. Step 5: Bond the bonding surface of the silicon layer (41) of the device to the upper surface of the cavity substrate; Step 6: Reduce the thickness of the substrate silicon layer (43); Step 7: Remove the remaining substrate silicon layer (43) and buried oxide layer (42) to expose the device silicon layer (41) and form a vibrating thin film (4). Step 8: Form an upper insulating layer (3) on the upper surface of the thin film (4); Step 9: Sputter metal and pattern an equipotential ring (2) on the upper insulating layer (3) and at the position corresponding to the pillar (5); Sputter metal and pattern an upper electrode (1) on the upper insulating layer (3) and at the position corresponding to the cavity (6), wherein the upper electrode (1) and the equipotential ring (2) are spaced apart from each other and electrically insulated; Step 10: Remove the second silicon dioxide layer (83) on the lower surface of the substrate (8) and use the exposed substrate (81) directly as the lower electrode.

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