A flexible piezoelectric micro-machined ultrasonic transducer array design and fabrication method

Abstract

The application relates to a flexible piezoelectric micro-machined ultrasonic transducer array design and preparation method, characterized in that N flexible plastic micro ultrasonic transducer units are arranged into an array, and two adjacent flexible plastic micro ultrasonic transducer units are connected together by a curved interconnection structure which is also a signal line for interconnection between the flexible plastic micro ultrasonic transducer units, and the flexible piezoelectric micro-machined ultrasonic transducer array has a large-amplitude bending function by using the curved interconnection structure. A flexible micro-electro-mechanical system (MEMS) ultrasonic transducer (MUT) array based on a thin film material can be prepared by the method, and the flexible micro-electro-mechanical system (MEMS) ultrasonic transducer (MUT) array can be used for detection and sensing of low-power environmental signals.

Description

Technical Field

[0001] This invention relates to a design and fabrication method for a flexible piezoelectric micromachining ultrasonic transducer (PMUT) array, belonging to the field of flexible micro-sensor technology. Background Technology

[0002] In recent years, with the development of technology, flexible electronic devices have shown great promise in wearable electronic devices, electronic skin, human-computer interaction, and robotics. Flexible sensors are a crucial component of flexible electronic devices. They are assembled using flexible conductive composite materials as a basis for circuit design. Under external stimuli such as force, temperature, light, and chemical signals, the electrical properties of the flexible sensor change, thereby achieving the response and sensing function of the applied signal.

[0003] With increasingly in-depth research into flexible sensors, flexible electronics has made outstanding contributions to human-computer interaction, health monitoring, wearable electronics, and artificial intelligence. In the application of flexible sensors, numerous physical signals, such as temperature, humidity, pressure, light, and some physiological parameters, are converted into electrical signals. Currently, there is much research on flexible mechanical sensors (capacitance / resistance), but limitations exist, including low sensing sensitivity, limited testing range, low repeatability of fabrication processes, and low repeatability of sample performance. The wide range of applications has led to continuously increasing demands on flexible sensors, with higher sensitivity being one of the requirements. In the field of flexible pressure sensors, those skilled in the art often use microfabrication processes to fabricate microstructures to improve sensitivity and reduce size. Initially, those skilled in the art made sensors flexible by transferring devices to flexible substrates and filling them with flexible media. Summary of the Invention

[0004] The technical problem to be solved by this invention is that existing flexible mechanical sensors have limitations such as low sensing sensitivity, limited testing range, low repeatability of processing and preparation processes, and low repeatability of sample performance.

[0005] To address the aforementioned technical problems, the present invention provides a method for designing and fabricating a flexible piezoelectric micromachining ultrasonic transducer array. The method is characterized by arranging N flexible and malleable micro-ultrasonic transducer units into an array, where N ≥ 3. Adjacent flexible and malleable micro-ultrasonic transducer units are connected by a curved interconnection structure, which also serves as the signal line connecting the flexible and malleable micro-ultrasonic transducer units. This curved interconnection structure enables the flexible piezoelectric micromachining ultrasonic transducer array to have a large bending capability.

[0006] Preferably, the flexible and malleable micro-ultrasonic transducer unit is prepared by the following steps:

[0007] Step 1: Prepare an SOI wafer.

[0008] Step 2: Deposit a releaseable material layer 1 on the SOI wafer, and then pattern the releaseable material layer 1 into a polygon shape by etching. The releaseable material layer 1 can be sacrificed.

[0009] Step 3: Deposit a second layer of releasable material on the portion of the SOI wafer not covered by the first layer of releasable material, and then pattern the second layer of releasable material into a polygonal shape by etching; the second layer of releasable material covers and is larger than the first layer of releasable material, has the same shape as the first layer of releasable material, and has a thickness less than the first layer of releasable material; the second layer of releasable material can be sacrificed.

[0010] Step 4: Deposit a structural layer on the SOIwafer, the first releasable material layer, and the second releasable material layer; a protrusion is naturally formed in the middle of the structural layer, and the shape and diameter of the protrusion are the same as those of the second releasable material layer.

[0011] Step 5: Make openings in the structural layer at the edge of the protruding portion of the structural layer to the second releasable material layer;

[0012] Step 6: By creating an opening, use any processing method to etch away the first and second releasable material layers, while retaining the structural layer, thereby forming a cavity beneath the structural layer;

[0013] Step 7: Seal the openings on the structural layer formed in Step 5 by any means to form an opening-sealed structure;

[0014] Step 8: Deposit a bottom electrode-piezoelectric-top electrode layer on the raised portion in the middle of the structural layer. The bottom electrode-piezoelectric-top electrode layer is located between the opening and sealing structures.

[0015] Step 9: Deposit a metal interconnect layer on the bottom electrode-piezoelectric-top electrode layer and the structural layer. The metal interconnect layer completely covers the portion of the bottom electrode-piezoelectric-top electrode layer and the structural layer located on one side of the bottom electrode-piezoelectric-top electrode layer, while the portion of the structural layer located on the other side of the bottom electrode-piezoelectric-top electrode layer is not covered by the metal interconnect layer. Pattern the metal interconnect layer into any shape by etching.

[0016] Step 10: Dry etch the substrate Si layer from the surface to the SOI wafer and pattern it;

[0017] Step 11: Remove the silicon dioxide layer and the substrate Si layer of the SOI wafer.

[0018] Preferably, when depositing the bottom electrode-piezoelectric-top electrode layer: first, a bottom electrode layer is deposited on the structural layer, and the bottom electrode layer is patterned into any shape by etching; then, a piezoelectric layer is deposited on the bottom electrode layer, and the piezoelectric layer is patterned into any shape by etching, the shape of the piezoelectric layer being the same as the shape of the bottom electrode layer; finally, a top electrode layer is deposited on the piezoelectric layer, and the top electrode layer is patterned into any shape by etching, the shape of the top electrode layer being the same as the piezoelectric layer and the bottom electrode layer.

[0019] Preferably, the flexible and malleable micro-ultrasonic transducer unit is prepared by the following steps:

[0020] Step 1: Prepare an SOI wafer;

[0021] Step 2: After etching a shallow trench on the upper surface of the SOIwafer, transfer the bottom electrode-piezoelectric-top electrode layer to the SOIwafer surface; the thickness of the shallow trench is no greater than the thickness of the top Si layer in the SOIwafer.

[0022] Step 3: First, deposit a metal interconnect layer on the surface of the bottom electrode-piezoelectric-top electrode layer, and then etch from the surface to the substrate Si layer of the SOI wafer and pattern it.

[0023] Step 4: Remove the silicon dioxide layer and the substrate Si layer from the SOI wafer.

[0024] Preferably, when forming the bottom electrode-piezoelectric-top electrode layer: prepare the bottom electrode layer by etching to pattern it into any shape; then deposit a piezoelectric layer on the bottom electrode layer by etching to pattern it into any shape, the shape of the piezoelectric layer being the same as the shape of the bottom electrode layer; finally deposit a top electrode layer on the piezoelectric layer by etching to pattern it into any shape, the shape of the top electrode layer being the same as the piezoelectric layer and the bottom electrode layer.

[0025] The method provided by this invention can fabricate a flexible microelectromechanical system (MEMS) ultrasonic transducer (MUT) array based on thin-film materials, which can be used for low-power environmental signal detection and sensing. The flexible mechanical sensor fabricated using the method provided by this invention has a higher sensor quality factor. Furthermore, because this invention uses materials with high electromechanical coupling coefficients, such as aluminum nitride or scandium aluminum nitride, and employs a special process design, the fabricated flexible mechanical sensor exhibits characteristics such as high array uniformity, low crosstalk, stress concentration, and large thin-film vibration displacement. Attached Figure Description

[0026] Figure 1A This is a schematic diagram of one structural form of the flexible piezoelectric micromachining ultrasonic transducer array disclosed in the embodiment;

[0027] Figure 1B This is a schematic diagram of another structural form of the flexible piezoelectric micromachining ultrasonic transducer array disclosed in the embodiment;

[0028] Figures 2A to 2K The fabrication process steps of the flexible and malleable micro-ultrasonic transducer unit disclosed in Example 1 are illustrated.

[0029] Figures 3A to 3D The fabrication process steps of the flexible and malleable micro-ultrasonic transducer unit disclosed in Example 2 are illustrated. Detailed Implementation

[0030] The present invention will be further illustrated below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Furthermore, it should be understood that after reading the teachings of this invention, those skilled in the art can make various alterations or modifications to the invention, and these equivalent forms also fall within the scope defined by the appended claims.

[0031] like Figure 1A As shown, the flexible piezoelectric micromachining ultrasonic transducer array disclosed in this embodiment is a 2×2 square array composed of four flexible and malleable micro-ultrasonic transducer units. Adjacent flexible and malleable micro-ultrasonic transducer units are connected together by an S-shaped metal wire, which also serves as the signal line interconnecting the flexible and malleable micro-ultrasonic transducer units. It should be noted that this embodiment uses an S-shaped metal wire as an example only; the present invention can use any curved interconnection structure instead. Figure 1A The S-shaped metal wires in the array. The interconnection structure with curved shapes, such as S-shaped metal wires, allows for relative degrees of freedom between adjacent flexible and malleable micro-ultrasonic transducer units, thus enabling the flexible piezoelectric micromachining ultrasonic transducer array to have a large bending capability.

[0032] Figure 1A The present invention is illustrated by taking only four flexible and malleable micro-ultrasonic transducer units as an example. After reading this patent, those skilled in the art should be able to conceive of obtaining a flexible piezoelectric micromachining ultrasonic transducer array by assembling an array of any number of flexible and malleable micro-ultrasonic transducer units. The arrangement of the malleable micro-ultrasonic transducer units in the array is not limited to... Figure 1A The square array shown can be any shape that a person skilled in the art can conceive of after reading this patent, such as a circle or a polygon. Figure 1B The structure of a flexible piezoelectric micromachining ultrasonic transducer array consisting of seven flexible and malleable micro-ultrasonic transducer units is illustrated.

[0033] The above-mentioned flexible and malleable micro-ultrasonic transducer unit can be prepared by the process method disclosed in Example 1 below, or by the process method disclosed in Example 2 below.

[0034] Example 1

[0035] This embodiment discloses a method for fabricating a flexible and malleable micro-ultrasonic transducer unit, which includes the following steps:

[0036] Step 1, as follows Figure 2A As shown, prepare an SOIwafer.

[0037] Step 2, as follows Figure 2B As shown, a releasable material layer 1 is deposited on an SOI wafer, and then patterned into a polygonal shape by etching. The releasable material layer 1 is sacrificial.

[0038] In this embodiment, the first releasable material layer is made of silicon dioxide, but other releasable materials can also be used. The first releasable material layer can be patterned into various polygonal shapes such as circles and squares.

[0039] Step 3, as follows Figure 2C As shown, a second releasable material layer is deposited on the portion of the SOI wafer not covered by the first releasable material layer. The second releasable material layer is then patterned into a polygonal shape by etching. The second releasable material layer surrounds the first releasable material layer, has the same shape as the first releasable material layer, and is thinner than the first releasable material layer. The second releasable material layer is sacrificial.

[0040] In this embodiment, the second releasable material layer is made of silicon dioxide, but other releasable materials can also be used. The second releasable material layer can be patterned into various polygonal shapes such as circles and squares.

[0041] Step 4, as follows Figure 2D As shown, a structural layer is deposited on the SOI wafer, the first releasable material layer, and the second releasable material layer. A protrusion is formed in the middle of the structural layer, and the shape and diameter of the protrusion are the same as those of the second releasable material layer.

[0042] In this embodiment, the structural layer is made of polycrystalline silicon or monocrystalline silicon, which has a higher selective etching ratio compared with the first and second release material layers.

[0043] Step 5, as follows Figure 2E As shown, the structural layer is perforated at the edge of the protruding portion of the structural layer to the second releasable material layer.

[0044] Step 6, as follows Figure 2F As shown, by creating an opening, the first and second releasable material layers are etched away using any processing method, while the structural layer is retained, thereby forming a cavity beneath the structural layer.

[0045] Step 7, as follows Figure 2GAs shown, the openings on the structural layer formed in step 5 are sealed by any means to form an opening-sealed structure.

[0046] Step 8, as follows Figure 2H As shown, a bottom electrode-piezoelectric-top electrode layer is deposited on the raised portion in the middle of the structural layer, located between the opening and sealing structures. During the deposition of the bottom electrode-piezoelectric-top electrode layer: first, a bottom electrode layer is deposited on the structural layer; the bottom electrode layer can be Pt or Au, etc., and is patterned into any shape through etching. Next, a piezoelectric layer is deposited on the bottom electrode layer; the piezoelectric layer can be aluminum nitride, scandium aluminum nitride, PZT, or lithium niobate, etc., and is patterned into any shape through etching, with the shape of the piezoelectric layer being the same as that of the bottom electrode layer. Finally, a top electrode layer is deposited on the piezoelectric layer; the top electrode layer can be Pt, Au, or Cr, etc., and is patterned into any shape through etching, with the shape of the top electrode layer being the same as that of the piezoelectric layer and the bottom electrode layer.

[0047] Step 9, as follows Figure 2I As shown, a metal interconnect layer is deposited on the bottom electrode-piezoelectric-top electrode layer and the structural layer. The metal interconnect layer completely covers the portion of the bottom electrode-piezoelectric-top electrode layer and the structural layer located on one side of the bottom electrode-piezoelectric-top electrode layer, while the portion of the structural layer located on the other side of the bottom electrode-piezoelectric-top electrode layer is not covered by the metal interconnect layer. The metal interconnect layer can be made of Au or Al, and can be patterned into any shape by etching.

[0048] Step 10, as follows Figure 2J As shown, the substrate Si layer of the SOI wafer is etched from the surface and patterned using any means.

[0049] Step 11, as follows Figure 2K As shown, the silicon dioxide layer and the substrate Si layer of the SOI wafer can be removed by any means, so that the upper device layer and the lower substrate layer are separated, while the upper device layer is retained.

[0050] Example 2

[0051] This embodiment discloses a method for fabricating a flexible and malleable micro-ultrasonic transducer unit, which includes the following steps:

[0052] Step 1, as follows Figure 3A As shown, prepare an SOIwafer.

[0053] Step 2, as follows Figure 3B As shown, after etching a shallow groove on the upper surface of the SOIwafer, the bottom electrode-piezoelectric-upper electrode layer is transferred to the SOIwafer surface.

[0054] The thickness of the shallow trench is no greater than the thickness of the top Si layer in the SOI wafer.

[0055] When forming the bottom electrode-piezoelectric-top electrode layer: Prepare the bottom electrode layer, which can be Pt or Au, etc., and pattern it into any shape by etching; then deposit a piezoelectric layer on the bottom electrode layer, which can be aluminum nitride, scandium aluminum nitride, PZT, or lithium niobate, etc., and pattern it into any shape by etching, with the shape of the piezoelectric layer being the same as that of the bottom electrode layer; finally, deposit a top electrode layer on the piezoelectric layer, which can be Pt, Au, or Cr, etc., and pattern it into any shape by etching, with the shape of the top electrode layer being the same as that of the piezoelectric layer and the bottom electrode layer.

[0056] Step 3, as follows Figure 3C As shown, a metal interconnect layer is first deposited on the surface of the bottom electrode-piezoelectric-top electrode layer, and then any means are used to etch from the surface to the substrate Si layer of the SOI wafer and pattern it.

[0057] Step 4, as follows Figure 3D As shown, the silicon dioxide layer and the substrate Si layer of the SOI wafer can be removed by any means, so that the upper device layer and the lower substrate layer are separated, while the upper device layer is retained.

Claims

1. A method of design and fabrication of a flexible piezoelectric micro-machined ultrasonic transducer array, characterized in that, N flexible plastic micro ultrasonic transducer unit array is arranged into an array, N ≥ 3, two adjacent flexible plastic micro ultrasonic transducer unit is connected together by the curved shape of the interconnection structure, the interconnection structure is also the signal line between the flexible plastic micro ultrasonic transducer unit interconnection, using the curved shape of the interconnection structure, so that the flexible piezoelectric micro ultrasonic transducer array has the function of large amplitude bending; The flexible plastic micro ultrasonic transducer unit is prepared by the following steps: Step 1, prepare an SOI wafer; Step 2, deposit a layer of releasable material layer one on the SOI wafer, then pattern the releasable material layer one into a polygonal shape by etching, and the releasable material layer one can be sacrificed; Step 3, deposit a layer of releasable material layer two on the part of SOI wafer not covered by the releasable material layer one, then pattern the releasable material layer two into a polygonal shape by etching; the releasable material layer two surrounds the releasable material layer one, and the shape and thickness of the releasable material layer two are the same as those of the releasable material layer one; the releasable material layer two can be sacrificed; Step 4, deposit a layer of structure layer on the SOI wafer, the releasable material layer one and the releasable material layer two; The middle part of the structure layer forms a protrusion, and the shape and diameter of the protrusion part are the same as those of the releasable material layer two; Step 5, open the structure layer to the releasable material layer two at the edge of the protrusion part of the structure layer; Step 6, by opening, etching away the releasable material layer one and the releasable material layer two by any processing means, and retaining the structure layer, thereby forming a cavity under the structure layer; Step 7, seal the opening of the structure layer formed in step 5 by any means to form an opening sealing structure; Step 8, deposit a bottom electrode-piezoelectric-upper electrode layer on the protrusion part of the middle part of the structure layer, and the bottom electrode-piezoelectric-upper electrode layer is located between the opening sealing structure; Step 9, deposit a layer of metal interconnection layer on the bottom electrode-piezoelectric-upper electrode layer and the structure layer, and the metal interconnection layer completely covers the part of the bottom electrode-piezoelectric-upper electrode layer and the structure layer on the side of the bottom electrode-piezoelectric-upper electrode layer, and the part of the structure layer on the other side of the bottom electrode-piezoelectric-upper electrode layer is not covered by the metal interconnection layer; pattern the metal interconnection layer into any shape by etching; Step 10, etch from the surface to the base Si layer of the SOI wafer and pattern; Step 11, remove the silicon dioxide layer and the base Si layer of the SOI wafer.

2. A flexible piezoelectric micromachined ultrasonic transducer array design and fabrication method as claimed in claim 1, wherein, When depositing the bottom electrode-piezoelectric-upper electrode layer: first, deposit a layer of bottom electrode layer on the structure layer, and pattern the bottom electrode layer into any shape by etching; then, deposit a layer of piezoelectric layer on the bottom electrode layer, and pattern the piezoelectric layer into any shape by etching, the shape of the piezoelectric layer being the same as that of the bottom electrode layer; finally, deposit a layer of upper electrode layer on the piezoelectric layer, and pattern the upper electrode layer into any shape by etching, the shape of the upper electrode layer being the same as that of the piezoelectric layer and the bottom electrode layer.

3. A flexible piezoelectric micromachined ultrasonic transducer array design and fabrication method as claimed in claim 1, wherein, The flexible plastic micro ultrasonic transducer unit is prepared by the following steps: Step 1, prepare an SOI wafer; Step 2, after etching a shallow trench on the surface of the SOI wafer, the bottom electrode-piezoelectric layer-top electrode layer is transferred to the surface of the SOI wafer; the thickness of the shallow trench is not greater than the thickness of the top Si layer in the SOI wafer; Step 3, after depositing a metal interconnection layer on the surface of the bottom electrode-piezoelectric layer-top electrode layer, the surface is etched to the base Si layer of the SOI wafer and is patterned; Step 4, the silicon dioxide layer and the base Si layer of the SOI wafer are removed.

4. A method of designing and fabricating a flexible piezoelectric micromachined ultrasonic transducer array as claimed in claim 3, wherein, When the bottom electrode-piezoelectric layer-top electrode layer is formed: the bottom electrode layer is prepared, the bottom electrode layer is patterned into any shape by etching; then a piezoelectric layer is deposited on the bottom electrode layer, the piezoelectric layer is patterned into any shape by etching, and the shape of the piezoelectric layer is the same as that of the bottom electrode layer; finally, a top electrode layer is deposited on the piezoelectric layer, the top electrode layer is patterned into any shape by etching, and the shape of the top electrode layer is the same as that of the piezoelectric layer and the bottom electrode layer.

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