Piezoelectric micromechanical ultrasonic transducer based on silicon carbide thin film and method for manufacturing same

Abstract

The present disclosure provides a piezoelectric micromechanical ultrasonic transducer based on a silicon carbide film and a preparation method thereof, which comprises a SiC layer, a top electrode layer, a bottom electrode layer and a cavity. The top electrode layer is arranged on the upper surface of the SiC layer, and the core comprises a center top electrode; the bottom electrode layer is arranged on the lower surface of the SiC layer; the cavity is used for releasing the diaphragm structure of the piezoelectric micromechanical ultrasonic transducer, so that the SiC layer is driven to vibrate and work based on its own piezoelectricity.

Description

Technical Field

[0001] This disclosure relates to the field of piezoelectric micromechanical ultrasonic transducers, and in particular to a piezoelectric micromechanical ultrasonic transducer based on silicon carbide thin film and its fabrication method. Background Technology

[0002] Currently, piezoelectric micromachined ultrasonic transducers (PMUTs) primarily use aluminum nitride (AlN) and lead zirconate titanate (PZT) as the piezoelectric functional layers. Aluminum nitride, with its high Curie temperature and good CMOS process compatibility, has been widely used in consumer electronics and biomedical imaging. Lead zirconate titanate, on the other hand, dominates in medical ultrasound and industrial sensing due to its extremely high piezoelectric coefficient. However, aluminum nitride exhibits significant piezoelectric response decay at high temperatures (>300℃), and lattice damage in strong radiation environments leads to irreversible performance degradation, failing to meet the demands of extreme environments such as aerospace and nuclear energy. Lead zirconate titanate contains the toxic heavy metal lead, is not biocompatible, and has a relatively low Curie temperature (~350℃), leading to depolarization at high temperatures and permanent device failure. Furthermore, lead zirconate titanate requires polarization treatment, and the polarization direction is prone to drift over time, raising concerns about long-term reliability. Both aluminum nitride and lead zirconate titanate exhibit varying degrees of stability issues in chemically corrosive environments (such as acidic cleaning solutions), limiting their lifespan in harsh industrial settings.

[0003] Ultrasonic transducers play a crucial role in non-destructive testing and condition monitoring in scenarios such as oil drilling, aero-engine monitoring, and nuclear reactor internal inspection. Currently, these applications mainly rely on bulk piezoelectric ceramic probes, which are large, power-hungry, and difficult to integrate into microsystems. Traditional bulk material transducers cannot be monolithically integrated with CMOS circuits, resulting in long signal links and high noise. Even AlN / PZT ultrasonic transducer devices manufactured using MEMS processes still face the risk of performance degradation or even functional loss under the aforementioned extreme conditions. There is currently no mature MEMS solution that can simultaneously meet the requirements of miniaturization, high reliability, and adaptability to extreme environments. Existing research on SiC MEMS devices mainly focuses on pressure sensors and accelerometers, and there are no reports on achieving efficient ultrasonic transmission and reception using SiC thin film vibration modes. Summary of the Invention

[0004] In view of the above, in order to at least partially solve at least one of the aforementioned technical problems, this disclosure provides a piezoelectric micromechanical ultrasonic transducer based on silicon carbide thin film and its fabrication method, the technical solution of which is as follows:

[0005] One embodiment of this disclosure provides a piezoelectric micromechanical ultrasonic transducer based on a silicon carbide thin film, comprising a SiC layer, a top electrode layer, a bottom electrode layer, and a cavity. The top electrode layer is disposed on the upper surface of the SiC layer and includes a central top electrode; the bottom electrode layer is disposed on the lower surface of the SiC layer; the cavity is used to release the diaphragm structure of the piezoelectric micromechanical ultrasonic transducer, allowing the SiC layer to drive the diaphragm structure to vibrate based on its own piezoelectricity.

[0006] According to an embodiment of this disclosure, the central top electrode is located on the upper surface of the SiC layer corresponding to the location of the cavity, and the center position of the central top electrode and the center position of the cavity are located in the same vertical direction.

[0007] According to an embodiment of this disclosure, the coverage area of ​​the central top electrode is greater than 10% of the cross-sectional area of ​​the cavity.

[0008] According to embodiments of this disclosure, the thickness of the bottom electrode layer is greater than the thickness of the SiC layer, so that the neutral axis of the diaphragm structure falls inside the bottom electrode layer; or the thickness of the central top electrode is greater than the thickness of the SiC layer, so that the neutral axis of the diaphragm structure falls inside the central top electrode.

[0009] According to embodiments of this disclosure, the shape of the cavity is selected from circles, ellipses, rectangles, and regular polygons.

[0010] According to embodiments of this disclosure, the SiC layer has intrinsic piezoelectricity, and the material used for preparation is selected from at least one of all crystal forms of SiC.

[0011] According to embodiments of this disclosure, the materials used to prepare the top electrode layer and the bottom electrode layer are selected from at least one of Al, Cr, Cu, Mo, Au, or non-metallic conductive materials.

[0012] According to an embodiment of this disclosure, the cavity is obtained by etching the substrate layer, the remaining substrate layer on the side of the cavity is used to provide support, and the remaining substrate layer on the upper side of the cavity is used to form part of the diaphragm structure. The substrate layer is a single-layer structure or a multi-layer structure.

[0013] According to embodiments of this disclosure, the substrate material is selected from silicon, silicon oxide, silicon-on-insulator, sapphire, or quartz.

[0014] Another embodiment of this disclosure provides a method for fabricating a piezoelectric micromechanical ultrasonic transducer based on a silicon carbide thin film, comprising: pretreating a substrate; fabricating and patterning a metal layer on the upper surface of the substrate to obtain a top electrode layer; fabricating a hard mask on the lower surface of the substrate and etching the lower part of the substrate using deep reactive ion etching to form a cavity structure; fabricating a bottom electrode layer on the lower surface of the SiC layer and fabricating an electrical connection structure for interconnecting the bottom electrode layer with the outside, thereby completing the fabrication of the piezoelectric micromechanical ultrasonic transducer based on a silicon carbide thin film. Attached Figure Description

[0015] The objects, features, and advantages of this disclosure will become clearer from the following description of embodiments of the present disclosure with reference to the accompanying drawings, in which:

[0016] Figure 1 This is a cross-sectional structural schematic diagram of a piezoelectric micromechanical ultrasonic transducer based on a silicon carbide thin film according to an embodiment of this disclosure.

[0017] Figure 2 This is a top view of the piezoelectric micromechanical ultrasonic transducer based on a silicon carbide thin film according to an embodiment of this disclosure.

[0018] Figure 3 This is a schematic diagram of the fabrication method of a piezoelectric micromechanical ultrasonic transducer based on a silicon carbide thin film according to an embodiment of this disclosure.

[0019] Figure 4 This is a schematic diagram of the testing method for a silicon carbide thin film-based piezoelectric micromechanical ultrasonic transducer according to an embodiment of this disclosure.

[0020] Figure 5 This is a cross-sectional structural schematic diagram of another piezoelectric micromechanical ultrasonic transducer based on silicon carbide thin film according to an embodiment of this disclosure.

[0021] Figure 6 This is a cross-sectional structural schematic diagram of another piezoelectric micromechanical ultrasonic transducer based on silicon carbide thin film according to an embodiment of the present disclosure. Detailed Implementation

[0022] This disclosure provides a piezoelectric micromechanical ultrasonic transducer based on silicon carbide thin film and its fabrication method. Using SiC thin film as the piezoelectric functional layer, its intrinsic piezoelectricity originates from the crystal structure itself, eliminating the need for polarization treatment and fundamentally avoiding the risk of depolarization. Simultaneously, its wide bandgap, high breakdown field strength, and high thermal conductivity make it naturally suitable for high-temperature, high-radiation, and corrosive environments. This invention is the first to apply SiC thin film to PMUT, combining the miniaturization advantages of MEMS devices with the extreme environmental tolerance of SiC materials, filling a technological gap in this field.

[0023] To make the objectives, technical solutions, and advantages of this disclosure clearer, the following detailed description is provided in conjunction with specific embodiments and the accompanying drawings.

[0024] In this embodiment of the disclosure, a piezoelectric micromechanical ultrasonic transducer based on silicon carbide thin film is provided, combined with Figure 1 and Figure 2As shown, the piezoelectric micromechanical ultrasonic transducer includes a SiC layer 1, a top electrode layer, a bottom electrode layer 5, and a cavity A. The top electrode layer is disposed on the upper surface of the SiC layer 1, and its core includes a central top electrode 3, and may also include an output plate electrode 4; the bottom electrode layer 5 is disposed on the lower surface of the SiC layer; the cavity is used to release the diaphragm structure 6 of the piezoelectric micromechanical ultrasonic transducer, so that the SiC layer 1 is driven by its own piezoelectricity to drive the diaphragm structure to vibrate.

[0025] According to the embodiments of this disclosure, the central top electrode 3 is located on the upper surface of the SiC layer 1 corresponding to the location of cavity A, and the center position of the central top electrode 3 and the center position of the cavity are located in the same vertical direction.

[0026] According to embodiments of this disclosure, the coverage area of ​​the central top electrode 3 is greater than 10% of the cross-sectional area of ​​the cavity. For example, the coverage area of ​​the central top electrode 3 can be 70%, 80%, 100%, or more than the cross-sectional area of ​​the cavity. It should be noted that this ratio can also be adjusted according to actual application conditions.

[0027] According to embodiments of this disclosure, since the suspended portion of the cavity structure can be used as a diaphragm structure, for example, the bottom electrode layer, the central top electrode, or other layers between the cavity and the SiC layer can all be part of the diaphragm structure. When the thickness of the bottom electrode layer 5 is greater than the thickness of the SiC layer 1, the neutral axis of the diaphragm structure 6 can fall inside the bottom electrode layer 5. Alternatively, when the thickness of the central top electrode is greater than the thickness of the SiC layer, the neutral axis of the diaphragm structure can fall inside the central top electrode.

[0028] According to embodiments of this disclosure, the shape of the cross-section of the cavity is selected from a circle, an ellipse, a rectangle, or a regular polygon, such as a square, a regular hexagon, or a regular octagon.

[0029] According to embodiments of this disclosure, the SiC layer 1 possesses intrinsic piezoelectricity, and the material used for its preparation is selected from any one of more than 250 crystal forms of SiC, preferably 4H-SiC, 6H-SiC, or 3C-SiC. The thickness of the SiC layer can be made in the nanometer or micrometer range depending on the specific application. It should be noted that the SiC layer film can be prepared on the substrate by transfer or film growth.

[0030] According to embodiments of this disclosure, the materials used to prepare the top electrode layer and the bottom electrode layer are selected from metallic materials such as Al, Cr, Cu, Mo, and Au, or from non-metallic conductive materials, composite conductive materials, etc., for example, conductive doped silicon. The doped silicon layer at the top of the SOI can serve as the bottom electrode layer. The thickness of the top electrode layer can be made from tens of nanometers to one or two hundred nanometers, for example, 200 nanometers, depending on the actual application. The thickness of the bottom electrode layer is several hundred nanometers or several micrometers, for example, 2 micrometers.

[0031] According to embodiments of this disclosure, the cavity is obtained by etching the substrate layer 2. The remaining substrate layer on the side of the cavity provides support, and the remaining substrate layer on the top of the cavity can be used to form part of the diaphragm structure. The substrate layer 2 can be a single-layer structure or a multi-layer structure. For example, the material used to prepare the substrate layer 2 is selected from silicon, silicon oxide, silicon-on-insulator (SOI), SOI with cavity (CSOI), silicon oxide wafer (silicon oxide on silicon), sapphire, or quartz. For substrate layers without a cavity structure, a sandwich structure of SiC+ upper and lower electrodes can be obtained by etching the substrate layer to release the diaphragm structure 6. If the substrate layer is an SOI with a cavity, the substrate layer itself has a cavity structure, and no additional etching processing is required; or a closed cavity structure can be formed by etching in the substrate layer.

[0032] In another embodiment of this disclosure, a method for fabricating the above-described piezoelectric micromechanical ultrasonic transducer based on silicon carbide thin film is also provided, such as... Figure 3 As shown, the preparation method includes the following operations or steps:

[0033] S1: Pretreatment of the substrate; taking 4H-SiCOI substrate as an example, such as... Figure 3 As shown, its top layer is SiC layer 1, and below the SiC layer is a substrate layer 2 composed of SiO2 layer 21 and Si layer 22;

[0034] S2: Prepare and pattern a metal layer on the upper surface of the substrate to obtain the top electrode layer (including the core central top electrode 3, and can also lead out the plate electrode 4).

[0035] S3: Prepare a hard mask on the lower surface of the substrate and etch the lower part of the substrate to form a cavity structure;

[0036] S4: Prepare a bottom electrode layer 5 on the lower surface of SiC layer 1, and prepare an electrical connection structure between the bottom electrode layer 5 and the outside to complete the fabrication of a piezoelectric micromechanical ultrasonic transducer based on silicon carbide thin film.

[0037] According to the embodiments of this disclosure, when etching the lower part of the substrate, any one of the following etching methods can be used: deep reactive ion etching (DRIE), KOH wet etching, TMAH etching, or XeF2 etching.

[0038] According to embodiments of this disclosure, the piezoelectric micromechanical ultrasonic transducer of the present invention can be further arrayed, that is, multiple piezoelectric micromechanical ultrasonic transducers are electrically connected in parallel to amplify signals. For arrayed connections, excitation signals of specific phases can be applied to different channels to achieve dynamic control of the ultrasonic beam.

[0039] Taking 4H-SiCOI substrate as an example, the fabrication process can specifically include: pretreatment of the 4H-SiCOI substrate, such as dicing and cleaning; depositing a 200 nm molybdenum layer on the surface of the 4H-SiCOI substrate using magnetron sputtering, further using photoresist as a mask, and using inductively coupled plasma (ICP) etching to etch the molybdenum layer to obtain the top electrode layer, wherein the photoresist pattern is processed using ultraviolet lithography; depositing a 200 nm chromium layer on the back of the substrate layer, and similarly using ultraviolet lithography combined with ICP etching to achieve patterning. This process mainly forms a hard mask for subsequent deep silicon etching. Compared with photoresist masks, metal hard masks have better masking effects and more precise control over cavity dimensions. Then, deep reactive ion etching (DRIE) is used to completely etch the silicon substrate and sacrificial layer to obtain the cavity structure, thereby releasing the diaphragm structure. Finally, a 2µm metal chromium layer is deposited from the back of the device using magnetron sputtering as the bottom electrode layer. The thickened metal chromium layer can also serve as the elastic structure layer of the diaphragm structure.

[0040] According to embodiments of this disclosure, the 4H-SiCOI substrate used in this case can be formed by transferring a single-crystal 4H-SiC thin film onto a wafer using a thin film transfer method (e.g., the Smart-Cut method).

[0041] According to the embodiments of this disclosure, the metal hard mask and the bottom electrode layer material used for back etching are both metallic chromium, and the bottom electrode layer is led out from the bottom of the device to achieve electrical connection with the outside.

[0042] According to the embodiments of this disclosure, a piezoelectric micromechanical ultrasonic transducer design was realized based on 4H-SiCOI. The principle is based on the intrinsic piezoelectricity of 4H-SiC. It should be noted that SiC has more than 250 crystal types, many of which do not have centrosymmetry and therefore have intrinsic piezoelectric effect. They can all be used as the core functional layer required for piezoelectric micromechanical ultrasonic transducers. For example, it can also be 6H-SiC or 3C-SiC material.

[0043] According to the embodiments of this disclosure, the metal materials of the back hard mask and the bottom electrode layer deposited last can be different. For example, they can be selected from metals such as Al, Cr, Cu, and Au. For example, the back etching hard mask uses metal Cr, while the bottom electrode layer metal uses Al. The back hard mask can also use materials such as SiO2 and Si3N4. Finally, the bottom electrode layer metal material is deposited, and the function of the bottom electrode leading out from the bottom can still be realized.

[0044] Methods for releasing the sacrificial layer include, but are not limited to, BOE solution etching, HF solution etching, HF gas etching, and RIE etching.

[0045] When testing piezoelectric micromechanical ultrasonic transducers, such as Figure 4As shown, the piezoelectric micromechanical ultrasonic transducer is bonded to the electrodes of the PCB circuit board 7 using conductive silver paste. Since the bottom electrode layer extends from the bottom of the device, the entire back of the device is attached to one electrode 8 on the PCB substrate. Additionally, the top electrode layer is bonded to another electrode 9 on the PCB substrate via wires 10 to differentiate the electrical connections and prevent short circuits between the upper and lower electrodes. Common materials for wire bonding include Al and Au. Besides conventional PCB substrates, glass substrates can also be used.

[0046] The bottom electrode layer extends from the bottom of the device to form an electrical connection structure for interconnection with the outside. At the same time, the bottom electrode layer at the corresponding position of the cavity also serves as the elastic layer of the piezoelectric micromechanical ultrasonic transducer. The cavity structure is used to release the diaphragm structure.

[0047] According to embodiments of this disclosure, such as Figure 1 As shown, a metal layer is prepared on the inner sidewall of cavity A and the bottom of the substrate layer to realize the electrical connection structure between the bottom electrode layer 5 and the outside.

[0048] According to embodiments of this disclosure, such as Figure 5 As shown, the substrate layer under the SiC layer 1 can also be silicon-on-insulator, that is, the substrate layer 2 from top to bottom includes a conductive doped Si layer 23 (as the bottom electrode layer), a SiO2 layer 24, and a Si layer 25. When etching the cavity structure from the substrate layer, it can be etched down to the lower surface of the SiO2 layer 24, so that the remaining SiO2 layer 24 can serve as the elastic layer of the diaphragm structure. At this time, vias 11 can be opened on the SiC layer 1 to expose the bottom electrode layer 5, and then the electrical connection structure between the bottom electrode layer 5 and the outside can be fabricated.

[0049] According to embodiments of this disclosure, such as Figure 6 As shown, the substrate layer under the SiC layer 1 can also be an SOI with a cavity. The cavity A is a closed cavity structure in the substrate layer 2. Similarly, a via 11 can be opened on the SiC layer 1 to expose the bottom electrode layer 5, and then an electrical connection structure for interconnection between the bottom electrode layer 5 and the outside can be prepared.

[0050] As can be seen, the substrate layer under the SiC layer can be selected according to the actual application. When it is necessary to etch the cavity, it can also be etched according to the actual application. The appropriate electrical connection structure of the bottom electrode layer can also be selected according to the actual application. For example, the bottom electrode layer can be electrically interconnected with the outside by leading out the electrical connection structure from the cavity A, or the bottom electrode layer can be electrically interconnected with the outside by opening a via in the SiC layer.

[0051] The embodiments of this disclosure have been described in detail above with reference to the accompanying drawings. It should be noted that implementations not illustrated or described in the drawings or the main text of the specification are forms known to those skilled in the art and have not been described in detail. It should be understood that the above are merely specific embodiments of this disclosure and are not intended to limit this disclosure. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this disclosure should be included within the protection scope of this disclosure.

Claims

1. A piezoelectric micromechanical ultrasonic transducer based on silicon carbide thin film, characterized in that, It includes a SiC layer, a top electrode layer, a bottom electrode layer, and a cavity: The top electrode layer is disposed on the upper surface of the SiC layer, including a central top electrode; The bottom electrode layer is disposed on the lower surface of the SiC layer; The cavity is used to release the diaphragm structure of the piezoelectric micromechanical ultrasonic transducer, so that the SiC layer can drive the diaphragm structure to vibrate based on its own piezoelectricity.

2. The piezoelectric micromechanical ultrasonic transducer based on silicon carbide thin film according to claim 1, characterized in that, The central top electrode is located on the upper surface of the SiC layer corresponding to the location of the cavity, and the center of the central top electrode and the center of the cavity are located in the same vertical direction.

3. The piezoelectric micromechanical ultrasonic transducer based on silicon carbide thin film according to claim 1, characterized in that, The coverage area of ​​the central top electrode is greater than 10% of the cross-sectional area of ​​the cavity.

4. The piezoelectric micromechanical ultrasonic transducer based on silicon carbide thin film according to claim 1, characterized in that, The thickness of the bottom electrode layer is greater than the thickness of the SiC layer, so that the neutral axis of the diaphragm structure falls inside the bottom electrode layer; or the thickness of the center top electrode is greater than the thickness of the SiC layer, so that the neutral axis of the diaphragm structure falls inside the center top electrode.

5. The piezoelectric micromechanical ultrasonic transducer based on silicon carbide thin film according to claim 1, characterized in that, The shape of the cavity is selected from circles, ellipses, rectangles, and regular polygons.

6. The piezoelectric micromechanical ultrasonic transducer based on silicon carbide thin film according to claim 1, characterized in that, The SiC layer has intrinsic piezoelectricity, and the material used to prepare it is selected from at least one of all crystal forms of SiC.

7. The piezoelectric micromechanical ultrasonic transducer based on silicon carbide thin film according to claim 1, characterized in that, The materials used to prepare the top electrode layer and the bottom electrode layer are selected from at least one of Al, Cr, Cu, Mo, Au, or non-metallic conductive materials.

8. The piezoelectric micromechanical ultrasonic transducer based on silicon carbide thin film according to claim 1, characterized in that, The cavity is obtained by etching the substrate layer. The remaining substrate layer on the side of the cavity is used to provide support, and the remaining substrate layer on the upper side of the cavity is used to form part of the diaphragm structure. The substrate layer is a single-layer structure or a multi-layer structure.

9. The piezoelectric micromechanical ultrasonic transducer based on silicon carbide thin film according to claim 8, characterized in that, The substrate is made of materials selected from silicon, silicon oxide, silicon-on-insulator, sapphire, or quartz.

10. A method for fabricating a piezoelectric micromechanical ultrasonic transducer based on a silicon carbide thin film according to any one of claims 1-9, characterized in that, include: Pre-treat the substrate; A metal layer is prepared and patterned on the upper surface of the substrate to obtain a top electrode layer; A hard mask is prepared on the lower surface of the substrate, and a cavity structure is formed by etching the lower part of the substrate using deep reactive ion etching. A bottom electrode layer is prepared on the lower surface of the SiC layer, and an electrical connection structure for interconnecting the bottom electrode layer with the outside is prepared to complete the fabrication of a piezoelectric micromechanical ultrasonic transducer based on silicon carbide thin film.

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