Center-edge bipolar wurtzite piezoelectric film enhanced stress sensor and preparation method thereof

Abstract

The invention discloses a center-edge bipolar wurtzite piezoelectric film enhanced stress sensor and a preparation method, and belongs to the technical field of electronic functional films, the center-edge bipolar wurtzite piezoelectric film enhanced stress sensor comprises an elastic substrate, an edge wurtzite piezoelectric film deposited on the edge of the elastic substrate, and a center wurtzite piezoelectric film deposited in the center of the elastic substrate, the device comprises an edge wurtzite piezoelectric film, a central wurtzite piezoelectric film, an edge electrode layer deposited on the edge wurtzite piezoelectric film, a central electrode layer deposited on the central wurtzite piezoelectric film, an insulating isolation layer deposited between the central electrode layer and the edge electrode layer, and a bridging lead located between the central electrode layer and the edge electrode layer. The growth parameters of the central wurtzite piezoelectric film and the edge wurtzite piezoelectric film are regulated and controlled, so that the polarization directions of the central wurtzite piezoelectric film and the edge wurtzite piezoelectric film are opposite, and when the elastic matrix generates elastic deformation under the action of external pressure, polarization charge changes of the central wurtzite piezoelectric film and the edge wurtzite piezoelectric film induced by deformation are superposed to form enhanced electric signals to be output. And the detection sensitivity of the stress sensor is greatly improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of electronic functional films, and relates to a wurtzite piezoelectric film for a thin film pressure sensor and a preparation method thereof, and in particular to a center-edge bipolar wurtzite piezoelectric film enhanced stress sensor and a preparation method thereof. Background Art

[0002] Thin-film stress sensors are devices that convert physical signals such as mechanical stress (such as vibration and air pressure) into electrical signals. Their core principle is based on the direct piezoelectric effect—when a piezoelectric material is subjected to an external force, its internal dipole moment changes, causing a change in surface charge density or potential difference, thereby outputting a detectable electrical signal. Due to their high sensitivity, fast response, and good linearity, piezoelectric thin-film sensors are widely used in aerospace, industrial automation, consumer electronics (such as touch screens and fingerprint recognition), medical devices (such as ultrasound imaging and blood pressure monitoring), and the Internet of Things (such as smart wearables and structural health monitoring).

[0003] Currently, mainstream thin-film stress sensors primarily utilize perovskite-type piezoelectric materials, such as lead zirconate titanate (PZT), lithium niobate (LN), and barium titanate (BT). However, these ferroelectric materials require high-field poling after thin-film deposition to align the internal electrical domains. However, high poling voltages can easily lead to film breakdown, reducing device yield and increasing process complexity. Wurtzite-structured piezoelectric materials, such as aluminum nitride (AlN) and zinc oxide (ZnO), have attracted widespread attention due to their unique spontaneous polarization properties. After deposition, these materials exhibit spontaneous polarization, with metal atoms pointing toward non-metal atoms. By manipulating defect concentrations and doping levels, the polarization direction can be selectively aligned, resulting in piezoelectric response without the need for additional pre-polarization. Furthermore, AlN and ZnO thin films exhibit excellent chemical stability, high breakdown field strength, and good CMOS process compatibility, making them ideal for applications such as MEMS sensors, RF filters, and energy harvesters.

[0004] A Chinese invention patent (CN114242884A) and existing research (Nam-In Kim et al., Adv. Funct. Mater. 2023, 33, 2212538) disclose single-layer thin-film sensors based on wurtzite-structured piezoelectric materials (such as ZnO and AlN). However, due to the limited polarization charge density of single-layer piezoelectric films, their output signal strength is low, making them difficult to meet the requirements of high-precision sensing applications.

[0005] In summary, the present invention deposits a piezoelectric film with a wurtzite structure on an elastic substrate, and controls the wurtzite films deposited at the center and edge of the substrate to have opposite polarities by regulating the film growth process parameters, and connects the central wurtzite piezoelectric film and the edge wurtzite piezoelectric film through a wire. This structure generates a stronger piezoelectric output signal when subjected to in-plane bending stress, and is suitable for the aerospace field for accurately measuring parameters such as air pressure and hydraulic pressure of aircraft. Summary of the Invention

[0006] In response to the limitations of conventional piezoelectric materials in the prior art and the research progress and existing problems of novel wurtzite materials, the present invention proposes a center-edge bipolar wurtzite piezoelectric film-enhanced stress sensor and its preparation method. While current research suggests that controlling sputtering process parameters to alter the initial polarization direction of wurtzite-structured films is a relatively mature approach, the present invention deposits wurtzite piezoelectric films of varying initial polarity onto an elastic substrate. Because wurtzite films possess high polarity without the need for prepolarization and generate charges in opposite directions on their surfaces when subjected to in-plane tensile stress, the highly polarized charges generated by the piezoelectric layers of varying polarity are detected, eliminating the need for prepolarization and achieving signal-enhanced stress detection.

[0007] In order to achieve the above object, the technical means adopted by the present invention are as follows:

[0008] An enhanced thin film stress sensor based on a center-edge bipolar wurtzite piezoelectric film is disclosed. The enhanced thin film stress sensor comprises an elastic substrate, an edge wurtzite piezoelectric film deposited on the edge of the elastic substrate, a central wurtzite piezoelectric film deposited in the center of the elastic substrate, an edge electrode layer deposited on the edge wurtzite piezoelectric film, a central electrode layer deposited on the center wurtzite piezoelectric film, an insulating isolation layer deposited between the central and edge electrode layers, and bridge leads between the central and edge electrode layers. The resulting enhanced thin film stress sensor is based on a center-edge bipolar wurtzite film. The central and edge wurtzite piezoelectric films have opposite polarities and are electrically connected via conductive lines. When the elastic substrate deforms under external pressure, the deformation-induced polarization charge changes on the central and edge wurtzite piezoelectric films are superimposed to produce an enhanced electrical signal output.

[0009] Based on the positive piezoelectric effect, under the action of tensile stress, the central wurtzite piezoelectric film and the edge wurtzite piezoelectric film will generate response polarization charges of different signs, thereby enhancing the output signal under the same mechanical pressure and improving the sensitivity of the enhanced thin film stress sensor.

[0010] Furthermore, the elastic substrate adopts a circular flat diaphragm structure with peripheral clamps. The elastic substrate is an elastic metal substrate that must be acid-resistant, heat-resistant, corrosion-resistant, high-strength, and tough, while also having excellent machinability and weldability. Possible materials include martensitic precipitation-hardening stainless steel and duplex stainless steel. After grinding and polishing, the surface of the elastic metal substrate is then sputtered with various thin films using magnetron sputtering equipment.

[0011] Furthermore, the edge wurtzite piezoelectric film, the central wurtzite piezoelectric and the insulating isolation layer constitute a piezoelectric sensing component.

[0012] Furthermore, the central wurtzite piezoelectric film is a central piezoelectric layer, and its material is wurtzite piezoelectric material, including wurtzite-based ZnO, AlN, GaN, and multi-doped compounds thereof, and Al is preferably used. x Sc 1-x N, Zn x Mg 1-x O, AlN, ZnO.

[0013] Furthermore, the edge wurtzite piezoelectric film is an edge piezoelectric layer, and its material is wurtzite piezoelectric material, including wurtzite-based ZnO, AlN, GaN, and multi-doped compounds thereof, and Al is particularly preferred. x Sc 1-x N, Zn x Mg 1-x O, AlN, ZnO.

[0014] Furthermore, the central wurtzite piezoelectric film and the edge wurtzite piezoelectric film are made of the same material, thereby improving process synergy and reliability.

[0015] Furthermore, by regulating the growth parameters of the central wurtzite piezoelectric film and the edge wurtzite piezoelectric film, the polarization directions of the two can be opposite.

[0016] Furthermore, the central electrode layer is selected from a high-conductivity metal material system, including: a high-conductivity pure metal, selected from at least one of Al, Cu, Ti, W, Ni, Ag, Au or Pt; a high-conductivity transition metal nitride material, selected from at least one of TiN, ZrN or HfN.

[0017] Furthermore, the edge electrode layer is selected from a high-conductivity metal material system, including: a high-conductivity pure metal, selected from at least one of Al, Cu, Ti, W, Ni, Ag, Au or Pt; a high-conductivity transition metal nitride material, selected from at least one of TiN, ZrN or HfN.

[0018] Furthermore, the central electrode layer and the edge electrode layer may be made of different materials.

[0019] Furthermore, the material of the insulating isolation layer is selected from SiO2, Al2O3 or other high dielectric strength insulating materials.

[0020] Furthermore, the bridge lead is made of a highly conductive metal material, including Cu, Ag, Au or Al, and is used to electrically connect the piezoelectric layer films with opposite polarization directions.

[0021] The present invention also provides a method for preparing a center-edge bipolar wurtzite piezoelectric film enhanced stress sensor, comprising the following steps:

[0022] Step S1, cleaning the elastic substrate: using ultraviolet irradiation + ultrasonic assisted cleaning process to remove metal particles, pollutants (including moisture and organic residues) and other impurities on the substrate surface. If necessary, electrochemical polishing is performed using a phosphoric acid-sulfuric acid mixture as an electrolyte and a voltage of 5-15 V to obtain an atomically flat surface.

[0023] Step S2, preparing a central piezoelectric layer: fixing the elastic substrate on a central polarity mask and transferring it to a vacuum sputtering chamber, and depositing a central piezoelectric layer thin film with a wurtzite structure using a magnetron sputtering process; by adjusting sputtering parameters, including but not limited to sputtering mode, power, working gas pressure, and gas flow rate, the deposited piezoelectric film exhibits a metal (e.g., Al / Zn) polarity to obtain a central piezoelectric layer thin film;

[0024] Step S3, preparing a central electrode layer: transferring the central wurtzite piezoelectric film deposited in the previous step to an electrode chamber, and depositing a central conductive thin film electrode as the central electrode layer using magnetron sputtering technology;

[0025] Step S4, preparing an edge piezoelectric layer: adjusting the elastic substrate to be fixed on an edge polarity mask, placing it in a vacuum sputtering chamber, and using a magnetron sputtering process to deposit an edge piezoelectric layer film with a wurtzite structure; by adjusting the sputtering parameters, including sputtering mode, power, working gas pressure, and gas flow, the deposited piezoelectric film exhibits a non-metallic (e.g., N / O) polarity and has the same film thickness as the central piezoelectric layer, ultimately obtaining an edge piezoelectric layer film;

[0026] Step S5, preparing edge electrodes: transferring the edge wurtzite piezoelectric film deposited in the previous step to an electrode chamber, and depositing edge conductive thin film electrodes using magnetron sputtering technology, with the film thickness being the same as that of the central conductive thin film electrode, to obtain an edge electrode layer, which is then removed after the device cools down;

[0027] Step S6, preparing an insulating isolation layer: fixing the device on a positioning fixture and placing them together in a vacuum sputtering chamber, and depositing an insulating isolation layer using magnetron sputtering technology;

[0028] Step S7, removing excess insulating isolation layer: Fix the stress sensor on a fixture, and then place it in a chemical mechanical polishing device to remove excess insulating isolation layer on the surface to expose the electrode film prepared in S3 and S5.

[0029] Step S8, wire bridging: using wires made of highly conductive materials to connect two wurtzite piezoelectric films with opposite polarities.

[0030] Furthermore, in step S2 and step S4, the sputtering process parameters for preparing the central piezoelectric layer and the edge piezoelectric layer can be interchanged, and ultimately it should be ensured that the polarization directions of the central wurtzite piezoelectric film and the edge wurtzite piezoelectric film are opposite.

[0031] Furthermore, the thickness of the central wurtzite piezoelectric film is 300~1500nm; the thickness of the central electrode layer is 50~200nm; the thickness of the edge wurtzite piezoelectric film is 300~1500nm; the thickness of the edge electrode layer is 50~200nm; and the thickness of the insulating isolation layer is 350~1700μm.

[0032] The principle of this invention is that when an elastic substrate is subjected to a certain pressure, it undergoes slight deformation, which results in in-plane tensile stress on the surface of the elastic substrate. By changing the thin film growth parameters of the wurtzite piezoelectric material, the polarity of the central and edge piezoelectric layers is reversed. Due to the opposite polarity, polarized surface charges of opposite signs are generated when subjected to stress. The magnitude of the stress to be measured is then determined by detecting the potential difference between the output terminals of the central and edge electrode layers, thereby achieving highly sensitive pressure sensing.

[0033] After adopting the above technical solution, the present invention has the following beneficial effects compared with the prior art:

[0034] (1) The present invention is based on the characteristics of strong preferential polarity of the wurtzite film after deposition, which eliminates the need for additional polarization steps and reduces the process complexity of the device.

[0035] (2) In traditional piezoelectric film structures with the same polarization direction, the film will generate polarized surface charges of the same polarity under stress, so stress can only be detected through a small difference in charge density, resulting in a weak output electrical signal. However, the present invention adjusts the initial polarization direction of the wurtzite piezoelectric film so that the spontaneous polarization directions of the central piezoelectric layer and the edge piezoelectric layer are opposite. Under the action of external stress, the central piezoelectric layer and the edge piezoelectric layer will bend and then be subjected to in-plane tensile stress, thereby forming polarized charges of opposite polarity on the surface of the central piezoelectric layer and the edge piezoelectric layer. This structural design can enhance the charge accumulation effect and produce a more significant electrical signal output under the same external excitation, thereby greatly improving the detection sensitivity of the stress sensor.

[0036] (3) Based on the above reasons, the present invention can be widely promoted in the field of piezoelectric film sensors. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 A cross-sectional view of the unipolar AlN piezoelectric film sensor used in Comparative Example 1; Figure 1 (a) is the main view of the unipolar AlN piezoelectric film sensor; Figure 1 (b) is a top view of a unipolar AlN piezoelectric film sensor (without the central electrode layer and the edge electrode layer).

[0038] Figure 2 This is a cross-sectional view of the sensor of the double-layer AlN piezoelectric thin film sensor used in Comparative Example 2.

[0039] Figure 3 A cross-sectional view of the sensor having a center-edge bipolar AlN piezoelectric film structure in Example 1; Figure 3 (a) is the main view of the sensor with a center-edge bipolar AlN piezoelectric film structure; Figure 3 (b) is a top view of the sensor with a center-edge bipolar AlN piezoelectric film structure.

[0040] Figure 4 A cross-sectional view of a sensor having a center-edge bipolar ZnO piezoelectric film structure in Example 2; Figure 4 (a) is the main view of the sensor with a center-edge bipolar ZnO piezoelectric film structure; Figure 4 (b) is a top view of the sensor with a center-edge bipolar ZnO piezoelectric film structure.

[0041] Figure 5 is the Al with center-edge bipolarity in Example 3 1-x Sc x N piezoelectric film sensor cross-sectional view; Figure 5 (a) is the center-edge bipolar Al 1-x Sc x The main view of the sensor of N piezoelectric film; Figure 5 (b) is the center-edge bipolar Al 1-x Sc x Top view of the N piezoelectric film sensor.

[0042] Figure 6 is the Zn with center-edge bipolarity in Example 4 1-x Mg x O Cross-sectional view of the sensor of piezoelectric film; Figure 6 (a) is the center-edge bipolar Zn 1-x Mg xO Main view of the piezoelectric film sensor; Figure 6 (b) is the center-edge bipolar Zn 1-x Mg x Top view of the piezoelectric film sensor.

[0043] Figure 7 The polarization surface charge density response characteristic curves of three piezoelectric film sensors, namely, comparative example 1 (unipolar AlN), comparative example 2 (AlN double layer) and embodiment 1 (center-edge bipolar AlN), under different mechanical pressures. DETAILED DESCRIPTION

[0044] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention more clear, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention.

[0045] Comparative Example 1

[0046] In this comparative example, a unipolar wurtzite AlN piezoelectric thin film sensor was prepared using magnetron sputtering technology, such as Figure 1 shown.

[0047] 1. Substrate pretreatment: 0Cr17Ni4Cu4Nb martensitic stainless steel was selected as the elastic substrate. It was first electropolished and then ultrasonically cleaned using acetone, anhydrous ethanol, and deionized water. After nitrogen drying, the substrate was placed in a central polarity mask device and transferred to a vacuum chamber.

[0048] 2. Preparation of the central piezoelectric layer: Using RF magnetron sputtering technology, Al was used as the target material. The target-elastic substrate spacing was set to 50 mm, the working pressure was 0.6 Pa, the RF power was 220 W, argon (50 SCCM) and nitrogen (10 SCCM) were introduced as working gases, and deposition was carried out at a substrate temperature of 350°C for 60 minutes to obtain a 1.0 μm thick AlN central piezoelectric layer with nitrogen polarity.

[0049] 3. Preparation of the central electrode layer: A DC magnetron sputtering system was used with Cu as the target material, a target-substrate distance of 65 mm, a working pressure of 0.7 Pa, a sputtering power of 150 W, an argon flow rate of 20 SCCM, and sputtering for 10 minutes at room temperature to form a 200 nm thick Cu central electrode layer.

[0050] 4. Preparation of edge piezoelectric layer: Using the same RF magnetron sputtering parameters as in step 2, a 1.0 μm thick AlN edge piezoelectric layer was deposited under the same process conditions, with its polarization direction consistent with that of the central piezoelectric layer.

[0051] 5. Preparation of edge electrode layer: Repeat the DC magnetron sputtering process parameters of step 3 to prepare a 200 nm thick Cu edge electrode layer on the surface of the edge piezoelectric layer.

[0052] 6. Preparation of insulating isolation layer: Pulsed DC magnetron sputtering technology was used, with Al as target material, target-substrate distance of 65mm, working pressure of 0.7Pa, power of 250W, argon (30SCCM) and oxygen (5SCCM), and deposition at room temperature for 60 minutes to form a 1.5μm thick SiO2 insulating isolation layer with a resistivity of 10 8 Ω level, ensuring good electrical insulation performance.

[0053] 7. Device post-processing: Fix the prepared sensor device on a special fixture, and remove the excess insulating isolation layer on the surface through a chemical mechanical polishing process to fully expose the contact area between the center electrode layer and the edge electrode layer.

[0054] 8. Use Cu wire to connect the center and edge electrode layer films to detect the potential difference between the two.

[0055] The piezoelectric coefficient of the prepared AlN film is 0.97pC / N. When the same stress is applied, the sensitivity of the unipolar AlN piezoelectric film sensor is relatively low.

[0056] Comparative Example 2

[0057] In this comparative example, the AlN double-layer piezoelectric thin film sensor was prepared by magnetron sputtering technology. Figure 2 shown.

[0058] 1. Substrate pretreatment: 0Cr17Ni4Cu4Nb martensitic stainless steel was selected as the elastic substrate. It was first electropolished and then ultrasonically cleaned using acetone, anhydrous ethanol, and deionized water. After nitrogen drying, the substrate was placed in a central polarity mask device and transferred to a vacuum chamber.

[0059] 2. Preparation of the first electrode layer: A DC magnetron sputtering process was used with Cu as the target material, a target-substrate distance of 65 mm, an operating pressure of 0.7 Pa, a sputtering power of 150 W, an argon flow rate of 20 SCCM, and sputtering at room temperature for 10 minutes to form a 200 nm thick Cu first electrode layer.

[0060] 3. Preparation of the first piezoelectric layer film: Using the RF magnetron sputtering process, Al was used as the target material, the target-elastic substrate distance was set to 50 mm, the working pressure was 0.6 Pa, the RF power was 220 W, argon (50 SCCM) and nitrogen (10 SCCM) were introduced as working gases, and the deposition was carried out at a substrate temperature of 350°C for 60 minutes to obtain a 1.0 μm thick AlN first piezoelectric layer film with Al polarity.

[0061] 4. Preparation of the second electrode layer: A DC magnetron sputtering process was used with Cu as the target material, a target-substrate distance of 65 mm, an operating pressure of 0.7 Pa, a sputtering power of 150 W, an argon flow rate of 20 SCCM, and sputtering at room temperature for 10 minutes to form a 200 nm thick Cu second electrode layer.

[0062] 5. Preparation of the second piezoelectric layer film: Using the RF magnetron sputtering process, Al was used as the target material, the target-elastic substrate spacing was set to 50 mm, the working pressure was 0.6 Pa, the RF power was 110 W, argon (50 SCCM) and nitrogen (10 SCCM) were introduced as working gases, and the deposition was carried out at a substrate temperature of 350°C for 100 minutes to obtain a 500 nm thick AlN second piezoelectric layer film with N polarity.

[0063] 6. Preparation of the third electrode layer: A 200 nm thick Cu third electrode layer was formed by using a DC magnetron sputtering process with Cu as the target material, a target-substrate distance of 65 mm, an operating pressure of 0.7 Pa, a sputtering power of 150 W, an argon flow rate of 20 SCCM, and sputtering at room temperature for 10 minutes.

[0064] 7. Use Cu wires to connect the first and third electrode layer films so that the two piezoelectric layer films are arranged in parallel.

[0065] Under the aforementioned film growth parameters, the piezoelectric coefficient of the first piezoelectric layer produced was 0.93pC / N, and the piezoelectric coefficient of the second piezoelectric layer was -0.86pC / N. However, due to the neutral plane between the first and second piezoelectric layers, the first piezoelectric layer generates tensile stress when subjected to mechanical pressure, while the second piezoelectric layer experiences compressive stress. This results in unequal polarization charges in the first and second piezoelectric layers, resulting in a loss of some piezoelectric signal, ultimately leading to low sensitivity in the AlN bilayer piezoelectric film sensor when detecting in-plane stress.

[0066] Example 1

[0067] In this embodiment, a center-edge bipolar wurtzite AlN piezoelectric thin film sensor is prepared by magnetron sputtering technology, such as Figure 3 shown.

[0068] 1. Substrate pretreatment: 0Cr17Ni4Cu4Nb martensitic stainless steel was selected as the elastic substrate. It was first electropolished and then ultrasonically cleaned using acetone, anhydrous ethanol, and deionized water. After nitrogen drying, the substrate was placed in a central polarity mask device and transferred to a vacuum chamber.

[0069] 2. Preparation of the central piezoelectric layer: RF magnetron sputtering was used with Al as the target material. The target-elastic substrate spacing was set to 50 mm, the working pressure was 0.6 Pa, the RF power was 220 W, argon (50 SCCM) and nitrogen (10 SCCM) were introduced as working gases, and the deposition was carried out at a substrate temperature of 350°C for 60 minutes to obtain a 500 nm thick AlN central piezoelectric layer with N polarity.

[0070] 3. Preparation of the central electrode layer: A DC magnetron sputtering system was used with Cu as the target material, a target-substrate distance of 65 mm, a working pressure of 0.7 Pa, a sputtering power of 150 W, an argon flow rate of 20 SCCM, and sputtering for 10 minutes at room temperature to form a 200 nm thick Cu central electrode layer.

[0071] 4. Preparation of edge piezoelectric layer: RF magnetron sputtering was used with Al as the target material. A target-elastic substrate spacing of 50 mm was set, an operating pressure of 0.6 Pa, an RF power of 110 W, and argon (50 SCCM) and nitrogen (10 SCCM) were introduced as working gases. Deposition was performed at a substrate temperature of 350°C for 100 minutes to obtain a 500 nm thick AlN edge piezoelectric layer with Al polarity.

[0072] 5. Preparation of edge electrode layer: Repeat the DC magnetron sputtering process parameters of step 3 to prepare a 200 nm thick Cu edge electrode layer on the surface of the edge piezoelectric layer.

[0073] 6. Preparation of insulating isolation layer: Pulsed DC magnetron sputtering technology was used, with Al as target material, target-substrate distance of 65mm, working pressure of 0.7Pa, power of 250W, argon (30SCCM) and oxygen (5SCCM), and deposition at room temperature for 60 minutes to form a 1.5μm thick SiO2 insulating isolation layer with a resistivity of 10 8 Ω level, ensuring good electrical insulation performance.

[0074] 7. Device post-processing: Fix the prepared sensor device on a special fixture, and remove the excess insulating isolation layer on the surface through a chemical mechanical polishing process to fully expose the contact area between the center electrode layer and the edge electrode layer.

[0075] 8. Use Cu wire to connect the center and edge electrode layer films to detect the potential difference between the two.

[0076] Under the aforementioned film growth parameters, the piezoelectric coefficient of the prepared central N-polarity AlN piezoelectric layer film is -0.87pC / N, and the piezoelectric coefficient of the edge Al-polarity piezoelectric layer film is 0.93pC / N. By varying the RF power of the RF sputtering, the polarization direction of the prepared AlN piezoelectric film can be altered, thereby eliminating the need for prepolarization. Furthermore, when the same stress is applied, the central and edge piezoelectric films generate approximately the same polarization charge. Therefore, the center-edge bipolar wurtzite piezoelectric film sensor has high sensitivity.

[0077] Example 2

[0078] In specific implementation, as a preferred implementation method of the present invention, a center-edge bipolar wurtzite ZnO piezoelectric thin film sensor is prepared by magnetron sputtering technology, such as Figure 4 shown.

[0079] 1. Substrate pretreatment: 0Cr17Ni4Cu4Nb martensitic stainless steel was selected as the elastic substrate. It was first electropolished and then ultrasonically cleaned using acetone, anhydrous ethanol, and deionized water. After nitrogen drying, the substrate was placed in a central polarity mask device and transferred to a vacuum chamber.

[0080] 2. Preparation of the central piezoelectric layer: RF magnetron sputtering was used with Zn as the target. The target-elastic substrate spacing was set to 30 mm, the working pressure was 0.6 Pa, the RF power was 200 W, and argon (20 SCCM) and oxygen (20 SCCM) were introduced as working gases. Deposition was performed at a substrate temperature of 400°C for 50 minutes to obtain a 400 nm thick ZnO central piezoelectric layer with O polarity.

[0081] 3. Preparation of the central electrode layer: A DC magnetron sputtering system was used with Pt as the target material, a target-substrate distance of 65 mm, an operating pressure of 0.4 Pa, a sputtering power of 100 W, an argon flow rate of 20 SCCM, and sputtering for 10 minutes at room temperature to form a 100 nm thick Pt central electrode layer.

[0082] 4. Preparation of edge piezoelectric layer: RF magnetron sputtering process was used, with Zn as the target material, the target-elastic substrate distance was set to 30mm, the working pressure was 0.6Pa, the RF power was 90W, argon (20SCCM) and nitrogen (20SCCM) were introduced as working gases, and the deposition was carried out at a substrate temperature of 400℃ for 120 minutes to obtain a 400μm thick ZnO edge piezoelectric layer with Zn polarity.

[0083] 5. Preparation of edge electrode layer: Repeat the DC magnetron sputtering process parameters of step 3 to prepare a 200 nm thick Pt edge electrode layer on the surface of the edge piezoelectric layer.

[0084] 6. Preparation of insulating isolation layer: Pulsed DC magnetron sputtering technology was used, with Si as target material, target-substrate distance of 65mm, working pressure of 0.7Pa, power of 250W, argon (20SCCM) and oxygen (10SCCM), and deposition at room temperature for 60 minutes to form a 2μm thick SiO2 insulating isolation layer with a resistivity of 10 8 Ω level, ensuring good electrical insulation performance.

[0085] 7. Device post-processing: Fix the prepared sensor device on a special fixture, and remove the excess insulating isolation layer on the surface through a chemical mechanical polishing process to fully expose the contact area between the center electrode layer and the edge electrode layer.

[0086] 8. Use Cu wire to connect the center and edge electrode layer films to detect the potential difference between the two.

[0087] Under the aforementioned film growth parameters, the piezoelectric coefficient of the prepared ZnO piezoelectric layer with central O polarity was -1.22pC / N, while the piezoelectric coefficient of the edge Zn polarity piezoelectric layer was 1.17pC / N. By varying the RF power during RF sputtering, the polarization direction of the prepared ZnO piezoelectric film can be altered, thus eliminating the need for prepolarization. Furthermore, under the same applied stress, the center-edge bipolar wurtzite piezoelectric film sensor exhibited high sensitivity.

[0088] Example 3

[0089] In specific implementation, as a preferred implementation method of the present invention, magnetron sputtering technology is used to prepare center-edge bipolar wurtzite Al x Sc 1-x N piezoelectric film sensors, such as Figure 5 shown.

[0090] 1. Substrate pretreatment: 0Cr17Ni4Cu4Nb martensitic stainless steel was selected as the elastic substrate. It was first electropolished and then ultrasonically cleaned using acetone, anhydrous ethanol, and deionized water. After nitrogen drying, the substrate was placed in a central polarity mask device and transferred to a vacuum chamber.

[0091] 2. Preparation of the central piezoelectric layer: Using RF magnetron sputtering technology, Al and Sc were used as target materials, the target-elastic substrate distance was set to 50 mm, the working pressure was 0.6 Pa, the RF power of the Al target was 185 W, and the RF power of the Sc target was 115 W. Argon (50 SCCM) and nitrogen (10 SCCM) were introduced as working gases. The deposition was carried out at a substrate temperature of 350 ° C for 60 minutes to obtain a 700 nm thick Al with N polarity. 1-x Sc x N central piezoelectric layer.

[0092] 3. Preparation of the central electrode layer: A DC magnetron sputtering system was used with Pt as the target material, a target-substrate distance of 65 mm, an operating pressure of 0.4 Pa, a sputtering power of 100 W, an argon flow rate of 20 SCCM, and sputtering for 10 minutes at room temperature to form a 100 nm thick Pt central electrode layer.

[0093] 4. Preparation of edge piezoelectric layer: RF magnetron sputtering process was used, with Al as target material, target-elastic substrate distance set to 50mm, working pressure 0.6Pa, RF power of Al target 185W, RF power of Sc target 75W, argon (50SCCM) and nitrogen (10SCCM) as working gas, deposition at 350℃ substrate temperature for 80 minutes, to obtain 700nm thick Al with Al polarity. 1-x Sc x N-edge piezoelectric layer.

[0094] 5. Preparation of edge electrode layer: Repeat the DC magnetron sputtering process parameters of step 3 to prepare a 100 nm thick Pt edge electrode layer on the surface of the edge piezoelectric layer.

[0095] 6. Preparation of insulating isolation layer: Pulsed DC magnetron sputtering technology was used, with Al as target material, target-substrate distance of 65mm, working pressure of 0.7Pa, power of 250W, argon (30SCCM) and oxygen (5SCCM), and deposition at room temperature for 60 minutes to form a 1.5μm thick SiO2 insulating isolation layer with a resistivity of 10 8 Ω level, ensuring good electrical insulation performance.

[0096] 7. Device post-processing: Fix the prepared sensor device on a special fixture, and remove the excess insulating isolation layer on the surface through a chemical mechanical polishing process to fully expose the contact area between the center electrode layer and the edge electrode layer.

[0097] 8. Use Cu wire to connect the center and edge electrode layer films to detect the potential difference between the two.

[0098] Under the above film growth parameters, the central N-polar Al 1-x Sc x The piezoelectric coefficient of the N piezoelectric layer film is -3.4pC / N, and the piezoelectric coefficient of the edge Al polarity piezoelectric layer film is 3.6pC / N. 1-x Sc x The Sc content in the N piezoelectric film can change the prepared Al 1-x Sc x The polarization direction of the N piezoelectric film is determined, thus achieving the concept of eliminating the need for pre-polarization. In addition, when the same stress is applied, the center-edge bipolar wurtzite piezoelectric film sensor has high sensitivity.

[0099] Example 4

[0100] In specific implementation, as a preferred embodiment of the present invention, magnetron sputtering technology is used to prepare center-edge bipolar wurtzite Zn 1-x Mg x O piezoelectric film sensors, such as Figure 6 shown.

[0101] 1. Substrate pretreatment: 0Cr17Ni4Cu4Nb martensitic stainless steel was selected as the elastic substrate. It was first electropolished and then ultrasonically cleaned using acetone, anhydrous ethanol, and deionized water. After nitrogen drying, the substrate was placed in a central polarity mask device and transferred to a vacuum chamber.

[0102] 2. Preparation of the central piezoelectric layer: RF magnetron sputtering was used with Zn and Mg as targets. The target-elastic substrate spacing was set to 30 mm, the operating pressure was 0.6 Pa, the RF power of the Zn target was 200 W, and the RF power of the Mg target was 100 W. Argon (20 SCCM) and oxygen (20 SCCM) were introduced as working gases. Deposition was performed at a substrate temperature of 400°C for 50 minutes to obtain a 600 nm thick ZnO central piezoelectric layer with O polarity.

[0103] 3. Preparation of the central electrode layer: A DC magnetron sputtering system was used with W as the target material, a target-substrate distance of 65 mm, a working pressure of 0.4 Pa, a sputtering power of 100 W, an argon flow rate of 20 SCCM, and sputtering for 15 minutes at room temperature to form a 100 nm thick W central electrode layer.

[0104] 4. Preparation of edge piezoelectric layer: RF magnetron sputtering was used with Zn and Mg as targets. The target-elastic substrate spacing was set to 30 mm, the operating pressure was 0.6 Pa, the RF power of the Zn target was 150 W, and the RF power of the Mg target was 75 W. Argon (20 SCCM) and oxygen (20 SCCM) were introduced as working gases. Deposition was performed at a substrate temperature of 400°C for 90 minutes to obtain a 600 nm thick ZnO edge piezoelectric layer with O polarity.

[0105] 5. Preparation of edge electrode layer: Repeat the DC magnetron sputtering process parameters of step 3 to prepare a 100 nm thick W edge electrode layer on the surface of the edge piezoelectric layer.

[0106] 6. Preparation of insulating isolation layer: Pulsed DC magnetron sputtering technology was used, with Si as target material, target-substrate distance of 65mm, working pressure of 0.7Pa, power of 250W, argon (20SCCM) and oxygen (10SCCM), and deposition at room temperature for 60 minutes to form a 2μm thick SiO2 insulating isolation layer with a resistivity of 10 8Ω level, ensuring good electrical insulation performance.

[0107] 7. Device post-processing: Fix the prepared sensor device on a special fixture, and remove the excess insulating isolation layer on the surface through a chemical mechanical polishing process to fully expose the contact area between the center electrode layer and the edge electrode layer.

[0108] 8. Use Cu wire to connect the center and edge electrode layer films to detect the potential difference between the two.

[0109] Under the above film growth parameters, the prepared central O polar Zn 1-x Mg x The piezoelectric coefficient of the O piezoelectric layer film is -1.63pC / N, and the piezoelectric coefficient of the edge Zn polarity piezoelectric layer film is 1.51pC / N. By changing the RF power of the RF sputtering, the prepared Zn 1-x Mg x The polarization direction of the O piezoelectric film is controlled, thus realizing the concept of eliminating the need for pre-polarization. In addition, when the same stress is applied, the center-edge bipolar wurtzite piezoelectric film sensor has high sensitivity.

[0110] The structural verification and analysis of Comparative Example 1 and Example 1 are as follows:

[0111] Since the device of the present application is a large-area wurtzite piezoelectric film structure deposited on an elastic substrate, when the elastic substrate is subjected to in-plane tensile stress, according to the positive piezoelectric effect equation (1), the polarization surface charge density of the central piezoelectric layer and the edge piezoelectric layer is:

[0112] (2)

[0113] (3)

[0114] Where, is the polarization surface charge density of the central piezoelectric layer, is the mechanical stress on the central piezoelectric layer, is the transverse piezoelectric coefficient of the central piezoelectric layer, is the polarization surface charge density of the edge piezoelectric layer, is the mechanical stress on the edge piezoelectric layer, is the transverse piezoelectric coefficient of the edge piezoelectric layer.

[0115] After the external mechanical stress is applied, the electric potential of the central piezoelectric layer and the edge piezoelectric layer is:

[0116] (4)

[0117] Where, is the electric potential of the piezoelectric layer, is the polarization surface charge density of the piezoelectric layer, is the thickness of the piezoelectric layer, is the vacuum dielectric constant, is the relative permittivity of the piezoelectric layer.

[0118] When subjected to external stress, since the central piezoelectric layer and the edge piezoelectric layer are connected in series, the potential difference output by the entire center-edge bipolar wurtzite piezoelectric film enhanced stress sensor is:

[0119] (5)

[0120] Where, is the potential difference or voltage between the central piezoelectric layer and the edge piezoelectric layer, and are the potentials of the central piezoelectric layer and the edge piezoelectric layer, and are the dielectric constants of the central piezoelectric layer and the edge piezoelectric layer, respectively, and t is the thickness of the central piezoelectric layer or the edge piezoelectric layer.

[0121] The difference between Comparative Example 1 and Example 1 is that the central piezoelectric layer and the edge piezoelectric layer of Comparative Example 1 have the same polarity, while the central piezoelectric layer and the edge piezoelectric layer of Example 1 have different polarities. The difference in the polarity of the piezoelectric layers gives the film transverse piezoelectric coefficients of opposite signs, which in turn causes the piezoelectric layers to generate opposite polarized surface charges when subjected to external stress. Therefore, for a stress sensor with the same polarity for the central piezoelectric layer and the edge piezoelectric layer, the detected voltage signal is simply the difference between the different numbers of polarized surface charges of the same sign. For a stress sensor with opposite polarity, the detected voltage signal is the difference between the different numbers of polarized surface charges of opposite signs, which will serve to enhance the electrical signal.

[0122] The structural verification and analysis of Comparative Example 2 and Example 1 are as follows:

[0123] For comparative example 2, due to the existence of neutral surfaces in the central piezoelectric layer and the edge piezoelectric layer, the first piezoelectric layer is subjected to in-plane tensile stress, and the second piezoelectric layer is subjected to in-plane compressive stress. According to the direct piezoelectric effect equation (1), the polarization surface charge density of the first piezoelectric layer and the second piezoelectric layer is:

[0124] (2)

[0125] (3)

[0126] Where, is the polarization surface charge density of the first piezoelectric layer, is the mechanical stress on the first piezoelectric layer, is the transverse piezoelectric coefficient of the first piezoelectric layer, is the polarization surface charge density of the edge wurtzite piezoelectric film, is the mechanical stress on the edge piezoelectric film, is the transverse piezoelectric coefficient of the edge piezoelectric film.

[0127] After the external mechanical stress is applied, the electric potential of the first piezoelectric layer and the second piezoelectric layer is:

[0128] (4)

[0129] Where, is the electric potential of the piezoelectric layer, is the polarization surface charge density of the piezoelectric layer, is the thickness of the piezoelectric layer, is the vacuum dielectric constant, is the relative permittivity of the piezoelectric layer.

[0130] When subjected to external stress, the potential difference output by the AlN double-layer piezoelectric film sensor is:

[0131] (5)

[0132] Where, is the potential difference or voltage between the first piezoelectric layer and the second piezoelectric layer, and are the potentials of the first piezoelectric layer and the second piezoelectric layer, and are the dielectric constants of the first piezoelectric layer and the second piezoelectric layer respectively, and t is the thickness of the first piezoelectric layer and the second piezoelectric layer.

[0133] The difference between Comparative Example 2 and Example 1 is that Comparative Example 2 is a double-layer AlN piezoelectric film sensor. Due to the presence of a neutral layer in the film, when subjected to mechanical pressure input, the first piezoelectric layer is subjected to compressive stress and the second piezoelectric layer is subjected to tensile stress. Since the stress signs of the first and second piezoelectric layers are opposite, and the signs of the transverse piezoelectric coefficients generated by the different polarities of the first and second piezoelectric layers are also opposite, the output potential difference or voltage signal is weak. In contrast, in Example 1 of the present application, for the center-edge bipolar wurtzite AlN piezoelectric film sensor, the stress signs of the first and second piezoelectric layers are the same, the signs of the transverse piezoelectric coefficients are opposite, and the final output potential difference or voltage signal is strong.

[0134] According to the test results of the piezoelectric tester, such as Figure 7As shown in the figure, under different mechanical pressure conditions, the polarization surface charge density (q) and the loading force (F) of the two piezoelectric film sensors in Comparative Example 1, Comparative Example 2, and Example 1 have an approximately linear relationship. Based on the definition of the sensitivity of a piezoelectric sensor, the sensitivity (S) of the piezoelectric film sensor of the present invention can be characterized by the following formula:

[0135] S = Δq / ΔF (6)

[0136] Where: S represents the sensitivity of the piezoelectric film sensor; Δq represents the change in polarization surface charge density; ΔF represents the change in applied mechanical load force.

[0137] Sensitivity is the slope of the two straight lines in the figure. The slope of the line showing the change of polarization surface charge density q with loading force F obtained in Example 1 is significantly higher than that in Comparative Examples 1 and 2.

[0138] Through the above verification analysis and Figure 7 (Characteristic curves of polarization surface charge density of two different piezoelectric film sensors under different mechanical pressure inputs in Comparative Examples 1 and 2, and Example 1) show that the center-edge bipolar piezoelectric film sensor shown in Example 1 has significantly higher sensitivity than the unipolar piezoelectric film sensors and the dual-layer piezoelectric film sensors in Comparative Examples 1 and 2. Depositing wurtzite piezoelectric films of different polarities at the center and edge of an elastic substrate can indeed enhance the performance of thin-film stress sensors, achieving increased sensitivity.

[0139] The above-described embodiments merely express the implementation methods of the present invention, but should not be understood as limiting the scope of the present invention. It should be pointed out that those skilled in the art can make several modifications and improvements without departing from the concept of the present invention, which all fall within the scope of protection of the present invention.

Claims

1. A center-edge bipolar wurtzite piezoelectric film enhanced stress sensor, characterized in that: The enhanced thin film stress sensor comprises: an elastic substrate, an edge wurtzite piezoelectric film deposited on the edge of the elastic substrate, a central wurtzite piezoelectric film deposited on the center of the elastic substrate, an edge electrode layer deposited on the edge wurtzite piezoelectric film, a central electrode layer deposited on the central wurtzite piezoelectric film, an insulating isolation layer deposited between the central electrode layer and the edge electrode layer, and bridge leads located between the central electrode layer and the edge electrode layer; The central wurtzite piezoelectric film and the edge wurtzite piezoelectric film have opposite polarities and are electrically connected through a conductive circuit. When the elastic matrix produces elastic deformation under the action of external pressure, the polarization charge changes induced by the deformation of the central wurtzite piezoelectric film and the edge wurtzite piezoelectric film are superimposed to form an enhanced electrical signal output.

2. The center-edge bipolar wurtzite piezoelectric film enhanced stress sensor according to claim 1, characterized in that: The elastic matrix is ​​a circular flat diaphragm structure, and the material is an elastic metal matrix.

3. The center-edge bipolar wurtzite piezoelectric film enhanced stress sensor according to claim 2, characterized in that: The elastic matrix material includes martensitic precipitation hardening stainless steel and duplex stainless steel.

4. The center-edge bipolar wurtzite piezoelectric film enhanced stress sensor according to claim 1, characterized in that: In the center-edge bipolar wurtzite piezoelectric film enhanced stress sensor: The central wurtzite piezoelectric film is a central piezoelectric layer, and its material is wurtzite piezoelectric material, including wurtzite-based ZnO, AlN, GaN and multi-doped compounds thereof; The edge wurtzite piezoelectric film is an edge piezoelectric layer, and its material is wurtzite piezoelectric material, including wurtzite-based ZnO, AlN, GaN and multi-doped compounds thereof; The central electrode layer is selected from a high-conductivity metal material system, including high-conductivity pure metals and high-conductivity transition metal nitride materials; The edge electrode layer is selected from a high-conductivity metal material system, including high-conductivity pure metals and high-conductivity transition metal nitride materials; The bridge lead is made of a highly conductive metal material.

5. The center-edge bipolar wurtzite piezoelectric film enhanced stress sensor according to claim 4, characterized in that: In the center-edge bipolar wurtzite piezoelectric film enhanced stress sensor: The material of the central wurtzite piezoelectric film is Al x Sc 1-x N, Zn x Mg 1-x O, AlN, ZnO; The material of the edge wurtzite piezoelectric film is Al x Sc 1-x N, Zn x Mg 1-x O, AlN, ZnO; The highly conductive pure metal in the central electrode layer is selected from at least one of Al, Cu, Ti, W, Ni, Ag, Au, or Pt; the highly conductive transition metal nitride material is selected from at least one of TiN, ZrN, or HfN; The high-conductivity pure metal in the edge electrode layer is selected from at least one of Al, Cu, Ti, W, Ni, Ag, Au or Pt; the high-conductivity transition metal nitride material is selected from at least one of TiN, ZrN or HfN; The bridge lead comprises Cu, Ag, Au or Al; The material of the insulating isolation layer is selected from SiO2, Al2O3 or other high dielectric strength insulating materials.

6. The center-edge bipolar wurtzite piezoelectric film enhanced stress sensor according to claim 1, characterized in that: The central wurtzite piezoelectric film and the edge wurtzite piezoelectric film are made of the same material, and their polarization directions are opposite by regulating the growth parameters of the central wurtzite piezoelectric film and the edge wurtzite piezoelectric film.

7. The center-edge bipolar wurtzite piezoelectric film enhanced stress sensor according to claim 1, characterized in that: The central electrode layer and the edge electrode layer may be made of different materials.

8. A method for preparing a center-edge bipolar wurtzite piezoelectric film enhanced stress sensor according to any one of claims 1 to 7, characterized in that: The preparation method comprises the following steps: Step S1, cleaning the elastic substrate: using ultraviolet irradiation + ultrasonic assisted cleaning process to clean the surface of the elastic substrate to obtain an atomically smooth surface; Step S2, preparing a central piezoelectric layer: fixing the elastic substrate on a central polarity mask and transferring it to a vacuum sputtering chamber, and depositing a central piezoelectric layer thin film with a wurtzite structure using a magnetron sputtering process; by adjusting the sputtering parameters, including sputtering mode, power, working gas pressure, and gas flow, the deposited piezoelectric film exhibits metallic polarity, thereby obtaining a central piezoelectric layer thin film; Step S3, preparing a central electrode layer: transferring the central wurtzite piezoelectric film deposited in the previous step to an electrode chamber, and depositing a central conductive thin film electrode as the central electrode layer using magnetron sputtering technology; Step S4, preparing an edge piezoelectric layer: adjusting the elastic substrate to be fixed on an edge polarity mask, placing it in a vacuum sputtering chamber, and using a magnetron sputtering process to deposit an edge piezoelectric layer film with a wurtzite structure; by adjusting the sputtering parameters, including sputtering mode, power, working gas pressure, and gas flow, the deposited piezoelectric film exhibits non-metallic polarity and has the same film thickness as the central piezoelectric layer, thereby finally obtaining an edge piezoelectric layer film; Step S5, preparing edge electrodes: transferring the edge wurtzite piezoelectric film deposited in the previous step to an electrode chamber, and depositing edge conductive thin film electrodes using magnetron sputtering technology, with the film thickness being the same as that of the central conductive thin film electrode, to obtain an edge electrode layer, which is then removed after the device cools down; Step S6, preparing an insulating isolation layer: fixing the device on a positioning fixture and placing them together in a vacuum sputtering chamber, and depositing an insulating isolation layer using magnetron sputtering technology; Step S7, removing the redundant insulating isolation layer; Step S8: bridging the leads to conduct the two wurtzite piezoelectric films with opposite polarities.

9. The method for preparing a center-edge bipolar wurtzite piezoelectric film enhanced stress sensor according to claim 8, characterized in that: In step S2 and step S4, the sputtering process parameters for preparing the central piezoelectric layer and the edge piezoelectric layer can be interchanged, and ultimately it should be ensured that the polarization directions of the central wurtzite piezoelectric film and the edge wurtzite piezoelectric film are opposite.

10. The method for preparing a center-edge bipolar wurtzite piezoelectric film enhanced stress sensor according to claim 8, characterized in that: The thickness of the central wurtzite piezoelectric film is 300-1500 nm; the thickness of the central electrode layer is 50-200 nm; the thickness of the edge wurtzite piezoelectric film is 300-1500 nm; the thickness of the edge electrode layer is 50-200 nm; and the thickness of the insulating isolation layer is 350-1700 μm.

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