A bimorph piezoelectric thin film piezoelectric ultrasonic transducer and a preparation method thereof

Abstract

A dual-piezoelectric thin-film piezoelectric ultrasonic transducer includes, from bottom to top, a substrate, a seed layer, a bottom electrode layer, a top electrode layer, an insulating layer, and a lead layer. The substrate, from bottom to top, includes a bottom silicon layer, a buried oxide layer, and a structural silicon layer. A cavity is formed in the center of the substrate, extending from the bottom silicon layer to the buried oxide layer and forming an arc shape within the structural silicon layer. Between the bottom electrode layer and the top electrode layer, from bottom to top, are a first piezoelectric layer and a second piezoelectric layer. The first piezoelectric layer ensures a high c-axis orientation for the growth of the second piezoelectric layer, thereby enabling the dual-piezoelectric layer to exhibit a higher dynamic amplitude under the same excitation voltage. This invention combines a method of stress-optimized device performance to effectively improve the dynamic amplitude of the device. Furthermore, the dual-piezoelectric thin film device exhibits a greater piezoelectric response than a single-piezoelectric thin film device under the same voltage excitation by adjusting the thin film stress during the manufacturing process to optimize the electromechanical coupling coefficient.

Description

Technical Field

[0001] This invention relates to the field of ultrasonic sensing technology, specifically to a dual piezoelectric thin film PMUT and its preparation method. Background Technology

[0002] MEMS (Micro Electromechanical Systems) has experienced widespread and rapid development since 1960, and has now become a cutting-edge interdisciplinary research field based on microelectronics technology. MEMS systems use sensitive elements such as capacitors, piezoelectric sensors, and resonators to sense external physical signals and convert them into quantifiable and processable electrical signals. This system includes various sensors such as pressure, acceleration, and ultrasound sensors. To achieve the commercialization of these sensor products, smaller size, lower cost, and better device performance are becoming increasingly important.

[0003] A piezoelectric micromachined ultrasonic transducer (PMUT) is a type of MEMS device. It consists of a thin-film stack suspended above a cavity and a substrate. The stack comprises an upper electrode, a piezoelectric layer, and a lower electrode. When a voltage is applied to the upper and lower electrodes, the piezoelectric layer in the electrode-covered area converts electrical energy into mechanical energy due to the inverse piezoelectric effect. This mechanical force interacts with the lever arm at the fixed boundary, generating a vertical displacement that causes the thin film to vibrate at a specific frequency, radiating ultrasonic waves into the surrounding medium. The device's frequency is primarily determined by the thickness and radius of the structural and piezoelectric layers; therefore, the desired resonant frequency can be obtained by designing the dimensions of these two layers. Simultaneously, the PMUT can also function as a receiver to receive ultrasonic waves. The sound waves cause deformation of the thin film, generating an electrical signal within the piezoelectric layer due to the piezoelectric effect. This signal can be utilized and processed for applications such as ultrasonic ranging, gesture recognition, and ultrasonic imaging. The better and more stable the device performance, the easier it is to develop the product. For example, in ultrasonic ranging, the farther the distance, the greater the attenuation of the echo signal. If the sound pressure level (SPL) of a single PMUT device can be increased, long-distance ultrasonic detection can be achieved. Since SPL is proportional to the device amplitude, the amplitude of the device must be increased to achieve a high SPL.

[0004] Chinese invention patent with announcement number CN107511318 discloses a "piezoelectric ultrasonic transducer and its preparation method". The elastic structure layer reduces the stiffness of the device, making the device more sensitive to stress and reducing the frequency linear range of the excitation voltage. However, the frequency of the device is prone to deviation during operation, resulting in poor device stability. Furthermore, the single-layer piezoelectric layer makes the amplitude and sound pressure generated by the device insufficient.

[0005] Chinese invention patent CN114890372 discloses "a design and fabrication method of a PMUT with isolation trenches". It employs a circular isolation trench structure with connecting bridges at all four ends, dividing the isolation trench into four equal segments to achieve independence for individual devices in the array. A drawback of this design is the small linewidth of the connecting bridges. When subjected to large excitation voltages or device vibration, the connecting bridges are prone to breakage, damaging the devices and resulting in poor physical performance. Summary of the Invention

[0006] To address the shortcomings of the existing technologies, this invention provides a dual piezoelectric thin film PMUT and its fabrication method. It employs a double-layer, tightly contacted piezoelectric thin film structure. The bottom piezoelectric thin film ensures high c-axis orientation during the growth of the upper piezoelectric thin film, improving film quality. Furthermore, the dual piezoelectric thin film can generate a higher piezoelectric response than traditional single-layer piezoelectric thin film devices. Simultaneously, by adjusting process parameters during the manufacturing process, the internal stress of the device is controlled within a certain range, optimizing device performance.

[0007] The technical solution of the present invention is as follows:

[0008] A dual piezoelectric thin-film piezoelectric ultrasonic transducer includes a substrate, a seed layer, a bottom electrode layer, a top electrode layer, an insulating layer, and a lead layer stacked sequentially from bottom to top. The substrate comprises a bottom silicon layer, a buried oxide layer, and a structural silicon layer from bottom to top. A cavity is formed in the center of the substrate, extending from the bottom silicon layer to the buried oxide layer and forming an arc shape within the structural silicon layer. A first piezoelectric layer and a second piezoelectric layer are disposed between the bottom electrode layer and the top electrode layer from bottom to top. The first piezoelectric layer ensures a high c-axis orientation for the growth of the second piezoelectric layer, thereby enabling the dual piezoelectric layers to have a higher dynamic amplitude under the same excitation voltage.

[0009] Furthermore, the first and second piezoelectric layers are made of the same material and are circular with a diameter of 60% to 80% of the cavity diameter.

[0010] Furthermore, the materials of the first piezoelectric layer and the second piezoelectric layer are selected as 1μmSc0.2AlN0.8+0.2μmSc0.096AlN0.904, wherein the thickness of the first piezoelectric layer is thicker than the thickness of the second piezoelectric layer.

[0011] Furthermore, the electromechanical coupling coefficient is optimized by adjusting the thin film stress during the process, including setting the gas flow ratio used for thin film deposition to 1:5; setting the temperature at 300°C when depositing the double piezoelectric layer and at 200°C when depositing the upper and lower electrodes; and pre-treating the wafer at 450°C for about 90 seconds before deposition.

[0012] Furthermore, the seed layer is made of scandium-doped aluminum nitride (Sc0.096AlN0.904), and is made of the same material as the first piezoelectric layer.

[0013] Furthermore, the bottom electrode layer is made of molybdenum (Mo), is circular in shape, and has a diameter smaller than that of the cavity.

[0014] Furthermore, the top electrode layer is made of molybdenum (Mo) and covers the second piezoelectric layer.

[0015] Furthermore, the insulating layer is made of silicon dioxide and covers the entire surface of the unit. Holes for lead wire connection are provided in the center and on both sides of the insulating layer. The diameter of the central hole is slightly smaller than the diameter of the top electrode layer, and the holes on both sides are positioned opposite the excess edge portion of the bottom electrode layer relative to the piezoelectric layer.

[0016] Furthermore, the lead layer is made of Al / Cu, and a portion of the lead layer is deposited and contacts the bottom electrode layer for lead-out, while another portion is deposited and contacts the top electrode layer for lead-out.

[0017] On the other hand, the present invention provides a method for preparing the above-mentioned dual piezoelectric thin-film piezoelectric ultrasonic transducer, characterized by comprising the following steps:

[0018] S1. Provide a substrate, and sequentially deposit a seed layer, a bottom electrode layer, a first piezoelectric layer, a second piezoelectric layer and a top electrode layer on the surface of the substrate;

[0019] S2. The top electrode layer, the second piezoelectric layer and the first piezoelectric layer are sequentially etched to pattern the surface of the product obtained in S1;

[0020] S3. Deposit silicon dioxide as an insulating layer on the surface of the product obtained in S2, and etch electrode vias;

[0021] S4. Deposit Al / Cu as a lead layer on the surface of the product obtained in S3 and etch it to pattern it to facilitate direct contact with the top and bottom electrodes;

[0022] S5. Etch away the silicon dioxide insulating layer on the surface of the product obtained in S4.

[0023] S6. A certain thickness of silicon dioxide is deposited on the surface of the product obtained in S5 as an insulating layer (7) to protect the lead layer (8).

[0024] S7. For the product obtained in S6, the silicon dioxide portion above the lead wire is etched to expose electrode holes that can be connected to external electrodes. Electrode holes for connecting to external power supply are etched on the lead wire surface.

[0025] S8. The back of the product obtained in S7 is etched, and the insulating silicon dioxide layer (7) on the upper surface of the top electrode is etched away to release the vibrating film so that the film undergoes an initial deformation of height h.

[0026] Compared with the prior art, the beneficial effects of the present invention are:

[0027] 1) The double-layer piezoelectric film structure with close contact ensures the high c-axis orientation of the upper piezoelectric film growth, thus improving the film quality.

[0028] 2) By combining the double-layer piezoelectric structure with the fabrication process, a certain amount of stress is generated inside the device to optimize the electromechanical coupling coefficient and effectively improve the device performance. Attached Figure Description

[0029] Figure 1 This is a schematic diagram of the structure of the dual piezoelectric thin-film piezoelectric ultrasonic transducer of the present invention;

[0030] Figure 2 This is a schematic diagram of the process flow for the dual piezoelectric thin-film piezoelectric ultrasonic transducer of the present invention;

[0031] Figure 3 The stress and neutral axis position variation trends obtained through simulation;

[0032] Figure 4 To simulate the relationship between the neutral axis position and the electromechanical coupling coefficient of the device, we can see the neutral axis position corresponding to the maximum electromechanical coupling coefficient. Then, from... Figure 1 The optimal stress value can be obtained;

[0033] Figure 5 The surface profile test image of the prepared stressed double piezoelectric thin film device;

[0034] Figure 6 A schematic diagram showing the phase and frequency test information of the fabricated stressed double piezoelectric thin film device;

[0035] Figure 7 A schematic diagram of the time-domain amplitude of the device under 1Vpp sinusoidal excitation. Detailed Implementation

[0036] The present invention will be further defined below with reference to the accompanying drawings and embodiments, but these should not be construed as limiting the scope of protection of the present invention.

[0037] Please see Figure 1 , Figure 1The figure shows a schematic diagram of the structure of the dual piezoelectric thin-film piezoelectric ultrasonic transducer of the present invention. The dual piezoelectric thin-film piezoelectric ultrasonic transducer includes, from bottom to top, a substrate 1, a seed layer 2, a bottom electrode layer 3, a top electrode layer 6, an insulating layer 7, and a lead layer 8. The substrate includes, from bottom to top, a bottom silicon layer 101, a buried oxide layer 102, and a structural silicon layer 103. A cavity 9 is formed in the center of the substrate, extending from the bottom silicon layer to the buried oxide layer, and is arc-shaped within the structural silicon layer. A first piezoelectric layer 4 and a second piezoelectric layer 5 are provided from bottom to top between the bottom electrode layer and the top electrode layer. The first piezoelectric layer ensures a high c-axis orientation for the growth of the second piezoelectric layer, thereby enabling the dual piezoelectric layers to have a higher dynamic amplitude under the same excitation voltage.

[0038] Example 1:

[0039] The PMUT device with -30MPa compressive stress is a dual piezoelectric thin film. In this embodiment, the substrate 1 is an SOI substrate. The first piezoelectric layer is Sc0.096AlN0.904, the second piezoelectric layer is Sc0.2AlN0.8, and h is the deformation height generated by the device under a certain compressive stress.

[0040] The design steps include the following:

[0041] ① Determine the structure (including material thickness dimensions): The upper and lower electrodes of this PMUT device are made of Mo, and the two piezoelectric thin films are made of ScAlN with a certain amount of Sc element doping, with thicknesses of (1μmSc) and (1μmSc) respectively. 0.2 AlN 0.8 +0.2μmSc 0.096 AlN 0.904 The first piezoelectric thin film (0.2 μm Sc). 0.096 AlN 0.904 This ensures the second piezoelectric film (1μmSc) is protected. 0.2 AlN 0.8 The substrate is a custom-polished SOI (Silicon-On-Insulator) with a high c-axis orientation. Other dimensional parameters are shown in Table 1.

[0042] Table 1

[0043] Material Top electrode (6) Dual piezoelectric thin film (4+5) Bottom electrode (3) Structural layer (103) Seed layer (2) Buried oxygen layer (102) Cavity (9) Radius(μm) 330 331 335 575 575 150 425 Thickness (μm) 0.2 1.2 0.2 4 0.05 1 400

[0044] ② Use finite element simulation to obtain the relationship between stress and the position of the neutral axis, then obtain the relationship between the position of the neutral axis and the electromechanical coupling coefficient, thereby determining the stress corresponding to the maximum electromechanical coupling coefficient, such as... Figure 3 and Figure 4 At this time, the corresponding neutral axis is located inside the structural layer (103) and 0.7 μm away from the upper surface of the structural layer. At this time, the optimal stress is -30 MPa, which is 30 MPa compressive stress.

[0045] ③ After polishing and cleaning the SOI substrate 1, the seed layer 2, bottom electrode layer 3, first piezoelectric layer (Sc0.096AlN0.904) 4, second piezoelectric layer (Sc0.2AlN0.8) 5, top electrode layer 6, insulating silicon dioxide layer 7, and lead layer 8 are deposited sequentially by magnetron sputtering. The substrate 1 is an SOI substrate, including a bottom cavity 9 and two side support portions, a buried oxide layer silicon dioxide 102, and a structural silicon layer 103. The above-mentioned optimal stress value can be obtained by adjusting the process parameters during the process; for example, the gas flow ratio used for thin film deposition is set to 1:5; the temperature is set at 300℃ when depositing the double piezoelectric layer, and the temperature is set at 200℃ when depositing the top and bottom electrodes; the wafer is pretreated at 450℃ for about 90s before deposition.

[0046] Figure 2 The figure shows a schematic diagram of the process flow for the dual piezoelectric thin-film piezoelectric ultrasonic transducer of the present invention. The fabrication method of the dual piezoelectric thin-film piezoelectric ultrasonic transducer, using MEMS technology, includes the following steps:

[0047] S1. On an SOI silicon wafer, a seed layer 2, a bottom electrode layer 3, a first piezoelectric layer 4, a second piezoelectric layer 5, and a top electrode 6 are deposited sequentially according to a certain growth process.

[0048] S2. The top electrode layer 6, the second piezoelectric layer 5, the first piezoelectric layer 4, and the bottom electrode layer 3 are etched on the surface of the product obtained in S1 to form a pattern.

[0049] S3. For the product obtained in S2, a certain thickness of silicon dioxide is deposited on the surface of the device as an insulating layer 7, and electrode vias are etched out.

[0050] S4. Deposit Al / Cu as lead layer 8 on the product obtained in S3 to facilitate direct contact with the top electrode 6 and the bottom electrode 3;

[0051] S5. Etch away the silicon dioxide of the insulating layer 7 on the surface of the device for the product obtained in S4.

[0052] S6. Deposit a certain thickness of insulating layer (silicon dioxide) 7 and protective lead layer 8 on the surface of the product obtained in S5.

[0053] S7. For the product obtained in S6, etch electrode holes for connecting to an external power supply on the lead surface.

[0054] S8. Etch the back of the product obtained in S7 and etch away the insulating silicon dioxide 7 on the upper surface of the top electrode to release the vibrating film so that the film undergoes an initial deformation of height h.

[0055] The cavity 9 provides space for the piezoelectric film to vibrate in the vertical direction. Its thickness extends from the bottom silicon 101 to the buried oxide layer silicon dioxide 102, and is formed using the currently mature deep silicon etching technology.

[0056] Among them, the buried oxide layer 102 is made of silicon dioxide and serves as the stop layer for the deep silicon etching process on the bottom silicon substrate.

[0057] The structural layer is made of silicon, and its thickness and radius are among the main factors that determine the device performance.

[0058] The top electrode 6 and the bottom electrode 3 are both made of molybdenum (Mo), and the electrode diameter accounts for 78% of the bottom cavity diameter, which provides the best transmission and reception performance.

[0059] The second piezoelectric layer 5 and the first piezoelectric layer 4 are made of 1 μm thick Sc0.2AlN0.8 and 0.2 μm thick Sc0.096AlN0.904, respectively. The bilayer piezoelectric material can provide a better piezoelectric response than the single-layer piezoelectric material. Under the same excitation voltage, the bilayer piezoelectric material has a higher dynamic amplitude. The first piezoelectric layer (Sc0.096AlN0.904) also provides a better film growth environment for the second piezoelectric layer (Sc0.2AlN0.8).

[0060] The insulating layer 7 is made of silicon dioxide and covers the entire surface of the device. Holes for lead wire connection are opened in the center and on both sides of the insulating layer 7. The openings are located directly opposite the excess edge of the bottom electrode layer 3 relative to the piezoelectric layer, which facilitates the connection of the lead wire to the bottom electrode layer 3.

[0061] Among them, the lead layer 8 is made of Al / Cu, with one side contacting the opening at the bottom electrode 3 and the other side contacting the opening at the top electrode 6. The positive and negative terminals of the external excitation voltage are respectively connected to apply a certain potential to both sides of the piezoelectric layer. Due to the inverse piezoelectric effect, the device will generate vertical vibration and emit ultrasonic waves.

[0062] The optimal stress value was theoretically obtained through finite element simulation. During device fabrication, process parameters were adjusted, and the structural layer (103) was finally released through deep silicon etching. The stress will manifestly cause the thin film to undergo a certain degree of initial deformation. The specific deformation value can be measured by a contour tester. Figure 4 As shown.

[0063] The phase and frequency of the double piezoelectric layer device with this initial deformation can be measured by an electrical probe station, such as... Figure 5 As shown, the phase values ​​initially indicate a high electromechanical coupling trend in this structure.

[0064] In this method, a 1Vpp square wave excitation is applied to the device at its resonant frequency, and the actual amplitude of the device is observed in the time domain using a laser Doppler velocimeter. Figure 6 As shown.

[0065] The fabrication process uses relatively conventional and mature MEMS technology, which does not require the development of new processes and can save on process costs.

[0066] This invention provides a method for fabricating a dual piezoelectric thin film PMUT device with initial deformation, which has the following beneficial effects:

[0067] This embodiment effectively improves the electromechanical coupling coefficient of PMUT devices. By adjusting the compressive stress to 30 MPa during the process, the fabricated device has a static profile displacement of 2.4 μm. Subtracting the 1.6 μm thickness of the device itself from the top surface of the structural layer to the top electrode, the convex height h on the film in a relatively flat state is 0.7 μm, achieving an electromechanical coupling coefficient of 4.42%. By setting the function generator frequency to the first-order resonant frequency of the device and exciting the device with a 1Vpp square wave AC, the dynamic displacement displayed in the time domain reaches 2.3 μm, effectively increasing the dynamic amplitude and solving the problem of low amplitude (500 nm - 1 μm) in existing PMUT devices.

[0068] The above are merely preferred embodiments of the present invention. However, the dual piezoelectric thin film structure includes, but is not limited to, the 1μmSc0.2AlN0.8+0.2μmSc0.096AlN0.904 described in this example. Meanwhile, the stress optimization device performance method includes, but is not limited to, the stress of -30MPa and the performance of electromechanical coupling coefficient described in this example.

Claims

1. A dual piezoelectric thin-film piezoelectric ultrasonic transducer, comprising, from bottom to top, a substrate, a seed layer, a bottom electrode layer, a top electrode layer, an insulating layer, and a lead layer, characterized in that, The substrate comprises, from bottom to top, a bottom silicon layer, a buried oxide layer, and a structural silicon layer. A cavity is formed in the center of the substrate, which is cut off from the bottom silicon layer to the buried oxide layer and is arc-shaped in the structural silicon layer. A first piezoelectric layer and a second piezoelectric layer are provided from bottom to top between the bottom electrode layer and the top electrode layer. The first piezoelectric layer can ensure the high c-axis orientation of the growth of the second piezoelectric layer, thereby enabling the double piezoelectric layer to have a higher dynamic amplitude under the same excitation voltage. The first piezoelectric layer uses 0.2μm Sc 0.096 AlN 0.904 The material, the second piezoelectric layer is selected from 1μm Sc 0.2 AlN 0.8 The material is circular, with a diameter of 60% to 80% of the cavity diameter; The electromechanical coupling coefficient is optimized by adjusting the thin film stress during the process, including setting the gas flow ratio to 1:5 when depositing the thin film; setting the temperature to 300℃ when depositing the double piezoelectric layer and 200℃ when depositing the upper and lower electrodes; and pre-treating the wafer at 450℃ for 90s before deposition.

2. The dual piezoelectric thin film piezoelectric ultrasonic transducer of claim 1, wherein, The material of the seed layer is Sc 0.096 AlN 0.904 and is consistent with the material of the first piezoelectric layer.

3. The dual piezoelectric thin-film piezoelectric ultrasonic transducer according to claim 1, characterized in that, The bottom electrode layer is made of molybdenum (Mo), is circular in shape, and has a diameter smaller than that of the cavity.

4. The dual piezoelectric thin film piezoelectric ultrasonic transducer of claim 1, wherein, The top electrode layer is made of molybdenum (Mo) and covers the second piezoelectric layer.

5. The dual piezoelectric thin film piezoelectric ultrasonic transducer of claim 1, wherein, The insulating layer is made of silicon dioxide and covers the entire surface of the unit. Holes for lead wire connection are provided in the center and on both sides of the insulating layer. The diameter of the central hole is slightly smaller than the diameter of the top electrode layer, and the holes on both sides are positioned opposite the excess edge of the bottom electrode layer relative to the piezoelectric layer.

6. The dual piezoelectric thin film piezoelectric ultrasonic transducer of claim 1, wherein, The lead layer is made of Al / Cu, and a portion of the lead layer is deposited and contacts the bottom electrode layer for lead-out, while another portion is deposited and contacts the top electrode layer for lead-out.

7. A method for preparing the dual piezoelectric thin-film piezoelectric ultrasonic transducer according to any one of claims 1-6, characterized in that, Includes the following steps: S1. Provide a substrate, and sequentially deposit a seed layer, a bottom electrode layer, a first piezoelectric layer, a second piezoelectric layer and a top electrode layer on the surface of the substrate; S2. The top electrode layer, the second piezoelectric layer and the first piezoelectric layer are sequentially etched to pattern the surface of the product obtained in S1; S3. Deposit silicon dioxide as an insulating layer on the surface of the product obtained in S2, and etch electrode vias; S4. Deposit Al / Cu as a lead layer on the surface of the product obtained in S3 and etch it to pattern it to facilitate direct contact with the top and bottom electrodes; S5. Etch away the silicon dioxide insulating layer on the surface of the product obtained in S4; S6. The silicon dioxide deposited on the surface of the product obtained in S5 is used as an insulating layer (7) to protect the lead layer (8). S7. For the product obtained in S6, the silicon dioxide portion above the lead wire is etched to expose the electrode hole for the external electrode. Electrode holes for connecting to an external power supply are etched on the lead wire surface. S8. Etch the back of the product obtained in S7 and etch away the silicon dioxide insulating layer on the upper surface of the top electrode to release the vibrating film so that the film undergoes an initial deformation of height h.

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