A lithium niobate-based surface acoustic wave temperature sensor and a preparation method thereof

Abstract

The application discloses a lithium niobate-based surface acoustic wave sensor and a preparation method thereof. The sensor is composed of an alumina lower protective layer, a lithium niobate piezoelectric layer, an AlN transition layer, an interdigital transducer (IDT), a reflective grating, and an alumina upper protective layer to form a sandwich multilayer structure. The interdigital transducer is made of a high-temperature-resistant metal. The device has a simple structure and can effectively monitor temperature. Through the synergistic design of the multilayer sandwich structure, the AlN transition layer and the high-temperature-resistant metal electrode, the device can adapt to a high-temperature complex environment, is wireless and passive, has strong stability, can widen the upper temperature limit of the lithium niobate-based temperature sensor, and effectively improves the performance and service life of the lithium niobate-based temperature sensor.

Description

Technical Field

[0001] This invention belongs to the field of surface acoustic wave sensor technology, specifically relating to a surface acoustic wave temperature sensor based on lithium niobate. Background Technology

[0002] Surface acoustic wave (SAW) temperature sensors offer advantages such as small size, fast response, high sensitivity, ease of integration, and wireless passive detection capabilities, making them valuable for applications in industrial equipment condition monitoring, energy and power systems, aerospace, equipment service environment sensing, and high-temperature structural health monitoring. These sensors typically utilize the characteristic that the propagation speed or resonant frequency of acoustic waves on the surface of piezoelectric materials shifts with temperature changes, establishing a correlation between the frequency signal and temperature to achieve temperature detection.

[0003] Lithium niobate, as a commonly used piezoelectric material, possesses a high electromechanical coupling coefficient, excellent acoustic-to-electric conversion capability, and mature micro / nano fabrication compatibility, thus it is widely used in surface acoustic wave (SAW) devices. However, under high-temperature or complex service environments, SAW temperature sensors based on lithium niobate still have certain limitations. First, the surface state and piezoelectric properties of the lithium niobate piezoelectric layer on the surface are prone to degradation, thus affecting the stability of SAW propagation. Second, metal electrode structures such as interdigital transducers and reflective gratings are susceptible to oxidation, diffusion, desorption, cracking, or conductivity degradation. Third, interfacial stress concentration and interface degradation easily occur between the metal electrodes and the piezoelectric material, thereby affecting the long-term reliability of the device. Summary of the Invention

[0004] The present invention aims to solve at least one of the following problems in existing lithium niobate-based surface acoustic wave temperature sensors when they operate for a long time in complex environments such as high temperature, oxidation, corrosion and thermal cycling: easy degradation of the lithium niobate piezoelectric layer, insufficient stability of the metal electrode, easy failure of the interlayer interface, and decreased sensor reliability.

[0005] To solve at least one of the above-mentioned technical problems, the present invention provides a surface acoustic wave temperature sensor based on lithium niobate, the surface acoustic wave temperature sensor comprising, from bottom to top, a lower alumina protective layer, a lithium niobate piezoelectric layer, an AlN transition layer, an electrode structure, and an upper alumina protective layer.

[0006] The lithium niobate piezoelectric layer serves as a surface acoustic wave (SAW) propagation medium, enabling the conversion between electrical signals and SAW. The AlN transition layer is disposed between the lithium niobate piezoelectric layer and the electrode structure to improve the interfacial bonding between the metal electrode and the lithium niobate piezoelectric layer, enhance the adhesion stability of the electrode structure under high temperature and thermal cycling conditions, and reduce the risk of interlayer degradation due to thermal mismatch or interfacial reactions.

[0007] The electrode structure includes an interdigital transducer and a reflective grating. The interdigital transducer is used to excite surface acoustic waves (SAWs) under the action of an applied electrical signal, and can convert the mechanical wave signal into an electrical signal through the inverse piezoelectric effect when the SAWs return. The reflective gratings are respectively disposed on both sides of the interdigital transducer to reflect the SAWs propagating to their locations, so that the SAWs form a stable propagation path or resonant structure inside the device, thereby enhancing the signal strength and frequency recognition capability of the device.

[0008] The interdigitated transducer and reflector grid preferably employ a composite metal structure consisting of a high-temperature resistant metal layer and an adhesive metal layer. The high-temperature resistant metal layer can be selected from one or more of platinum, tungsten, and molybdenum, while the adhesive metal layer can be selected from one or more of titanium, tantalum, and chromium. The high-temperature resistant metal layer improves the conductivity stability of the electrode under high-temperature conditions, while the adhesive metal layer enhances the bonding strength between the electrode and the AlN transition layer or the underlying functional layer.

[0009] The reflective grating and the interdigital transducer use the same or similar metal material system. They can use the same metal system, or they can use metal systems with a linear thermal expansion coefficient difference of no more than 5 × 10⁻⁶. -6 The metal or alloy system of / K is used to ensure that the reflective grating and interdigital transducer have similar process compatibility and thermomechanical stability during deposition, patterning, and subsequent thermal cycling, thereby avoiding problems such as residual stress, interface delamination, or inconsistent acoustic response caused by excessive differences in material systems.

[0010] The upper alumina protective layer covers the interdigital transducer, reflector grating, and surface acoustic wave (SAW) propagation area, providing environmental isolation and protection for the electrode structure and SAW functional propagation area, reducing the impact of high-temperature oxidation, corrosive media, and particle erosion on the device's functional layers. The upper alumina protective layer can be a dense, continuous thin film, formed through magnetron sputtering, atomic layer deposition, chemical vapor deposition, physical vapor deposition, or other thin film deposition methods.

[0011] In a preferred embodiment, the upper alumina protective layer has a windowed area corresponding to the electrode pads, lead terminals, or antenna connection terminals to ensure electrical connection, signal readout, or wireless coupling of the device. The thickness of the upper alumina protective layer can be selected according to the surface acoustic wave wavelength, device operating frequency, and environmental protection requirements; preferably, the thickness of the upper alumina protective layer is less than 0.3 times the surface acoustic wave wavelength to reduce its overloading on surface acoustic wave propagation.

[0012] The lower alumina protective layer is disposed below the lithium niobate piezoelectric layer, providing mechanical support, back-side environmental isolation, and thermal shock protection for the lithium niobate piezoelectric layer. Through the synergistic effect of the upper alumina protective layer, the lower alumina protective layer, the AlN transition layer, the lithium niobate piezoelectric layer, and the high-temperature resistant electrode structure, this surface acoustic wave temperature sensor can maintain good structural stability and signal output reliability in high-temperature and complex environments.

[0013] The present invention also provides a method for fabricating the above-mentioned surface acoustic wave temperature sensor based on lithium niobate, comprising the following steps:

[0014] First, a lower alumina support protective layer is provided, and a single-crystal lithium niobate piezoelectric layer is disposed, bonded, or attached to the lower alumina support protective layer. Second, an aluminum nitride transition layer is formed on the side of the single-crystal lithium niobate piezoelectric layer away from the lower alumina support protective layer. Then, an adhesive metal layer and a high-temperature resistant metal layer are formed on the aluminum nitride transition layer, and an interdigital transducer and a reflective grid located on both sides of the interdigital transducer are formed through a patterning process. Subsequently, an upper alumina protective layer is formed on the interdigital transducer, the reflective grid, and the surface acoustic wave propagation region. Afterward, a window area is formed or retained at the corresponding positions of the electrode pads, lead ends, or antenna connection ends. Finally, the device is packaged, calibrated for temperature and frequency, and tested to establish the correspondence between the resonant frequency and temperature.

[0015] The aluminum nitride transition layer can be formed by magnetron sputtering, reactive magnetron sputtering, atomic layer deposition, chemical vapor deposition, or physical vapor deposition; the upper alumina protective layer can be formed by atomic layer deposition, magnetron sputtering, chemical vapor deposition, or physical vapor deposition; the interdigitated transducer and reflective grating can be formed by photolithography, metal deposition, and lift-off processes, or by metal deposition, photolithography, and etching processes.

[0016] The thicknesses of each layer are as follows: top alumina layer 150-250nm, high-temperature resistant metal layer 160-200nm, adhesive layer 10-30nm, AlN layer 80-120nm, lithium niobate piezoelectric layer 300-500μm, and bottom alumina layer 300-600μm.

[0017] Existing technologies offer several solutions to improve the environmental adaptability of surface acoustic wave (SAW) sensors, but these often struggle to simultaneously address multiple requirements, including piezoelectric layer protection, electrode stability, interface bonding, and temperature signal transmission. This invention protects the interdigital transducer, reflector grating, and SAW propagation region while ensuring effective transfer of external temperature to the SAW functional area. Furthermore, it compensates for initial frequency shifts or acoustic loading effects caused by SAW propagation through structural design and temperature-frequency calibration, thereby achieving stable temperature monitoring in high-temperature and complex environments.

[0018] Specifically, compared with the prior art, the present invention has at least the following beneficial effects:

[0019] First, by setting aluminum oxide protective layers on both the upper and lower sides of the lithium niobate piezoelectric layer, the present invention forms a multi-layered protective surface acoustic wave temperature sensing structure, which can reduce the direct impact of high temperature, oxidation, corrosion and particle erosion environment on the piezoelectric functional layer and metal electrode structure.

[0020] Secondly, the present invention provides an AlN transition layer between the lithium niobate piezoelectric layer and the electrode structure, which is beneficial to improve the interfacial bonding state between the metal electrode and the lithium niobate piezoelectric layer, reduce the risk of interfacial cracking, desorption or degradation under thermal cycling conditions, and improve the structural stability of the device.

[0021] Third, the present invention uses a high-temperature resistant metal layer and an adhesive metal layer to form an interdigital transducer and a reflective grid, which helps to improve the conductivity stability, adhesion stability and acoustic consistency of the electrode structure under high temperature environment.

[0022] Fourth, the upper alumina protective layer of this invention does not block the measured temperature, but rather transfers the external temperature to the surface acoustic wave functional region through thermal conduction. The initial frequency shift caused by the upper alumina protective layer can be used as a device structural parameter and compensated for through temperature-frequency calibration, thereby maintaining the temperature detection function while achieving environmental protection.

[0023] Fifth, the present invention improves the long-term reliability and application adaptability of the lithium niobate-based surface acoustic wave temperature sensor in high-temperature and complex environments through the synergistic design among the lower alumina protective layer, the lithium niobate piezoelectric layer, the AlN transition layer, the high-temperature resistant electrode structure, and the upper alumina protective layer. Attached Figure Description

[0024] Figure 1 This is a schematic diagram of the surface acoustic wave temperature sensor based on lithium niobate according to the present invention; wherein, 1 is the upper alumina protective layer, 2 is the interdigital transducer, 3 is the right-end reflective grating, 4 is the AlN transition layer, 5 is the lithium niobate piezoelectric layer, 6 is the lower alumina protective layer, and 7 is the left-end reflective grating.

[0025] Figure 2 This is a two-dimensional front view of a portion of the structure of the surface acoustic wave temperature sensor based on lithium niobate according to the present invention.

[0026] Figure 3 This is a process flow diagram of the fabrication process of the surface acoustic wave temperature sensor based on lithium niobate according to the present invention.

[0027] Figure 4 This is a schematic diagram of the temperature-frequency response relationship of the surface acoustic wave temperature sensor based on lithium niobate according to the present invention. Detailed Implementation

[0028] The specific embodiments of the present invention will be described below with reference to the accompanying drawings. It should be understood that the following embodiments are only used to further illustrate the technical solutions of the present invention and should not be construed as limiting the scope of protection of the present invention. Based on the disclosure of the present invention, equivalent substitutions, conventional improvements, or combinations thereof obtained by those skilled in the art without inventive effort should all fall within the scope of protection of the present invention.

[0029] like Figure 1 and Figure 2 As shown, this embodiment provides a surface acoustic wave temperature sensor based on lithium niobate, including an upper alumina protective layer 1, an interdigital transducer 2, a right-end reflective grating 3, an AlN transition layer 4, a lithium niobate piezoelectric layer 5, a lower alumina protective layer 6, and a left-end reflective grating 7.

[0030] The lower alumina protective layer 6 is located at the bottom of the device and provides mechanical support and back-side environmental isolation for the lithium niobate piezoelectric layer 5. The lower alumina protective layer 6 exhibits good high-temperature resistance and chemical stability, reducing the risk of the back side of the device being affected by high-temperature environments, oxidizing environments, or corrosive media, and contributing to improved overall device structural stability. The thickness of the lower alumina protective layer 6 is preferably in the range of 300-600 μm.

[0031] A lithium niobate piezoelectric layer 5 is disposed above the lower alumina protective layer 6, serving as the piezoelectric dielectric layer for surface acoustic wave (SAW) propagation. When the interdigital transducer 2 receives an external electrical signal, the lithium niobate piezoelectric layer 5 can excite SAW propagation in its surface or near-surface region through the piezoelectric effect; when the SAW returns to the region where the interdigital transducer 2 is located, it can be converted into an electrical signal output through the inverse piezoelectric effect. The thickness of the lithium niobate piezoelectric layer 5 is preferably in the range of 300-500 μm.

[0032] An AlN transition layer 4 is disposed between the lithium niobate piezoelectric layer 5 and the interdigital transducer 2, the right-end reflective grating 3, and the left-end reflective grating 7. Specifically, the interdigital transducer 2, the right-end reflective grating 3, and the left-end reflective grating 7 are located on the upper surface of the AlN transition layer 4, while the lithium niobate piezoelectric layer 5 is located on the lower surface of the AlN transition layer 4. The AlN transition layer 4 improves the interfacial bonding between the metal electrode structure and the lithium niobate piezoelectric layer 5, reducing the risk of desorption, cracking, or interfacial degradation caused by differences in material thermal expansion, interfacial stress concentration, or interfacial reactions under high-temperature thermal cycling conditions. Simultaneously, AlN material possesses good high-temperature stability and dielectric properties, making it suitable as a transitional functional layer between the lithium niobate piezoelectric layer 5 and the metal electrode structure. The thickness of the AlN transition layer 4 is preferably in the range of 80-120 nm. In this embodiment, the interdigital transducer 2 is composed of multiple pairs of interlaced metal interdigitates, used to achieve the mutual conversion between electrical signals and surface acoustic waves. The right-end reflector grating 3 and the left-end reflector grating 7 are respectively disposed on both sides of the interdigital transducer 2, and together with the interdigital transducer 2, form a surface acoustic wave propagation or resonance structure. The right-end reflector grating 3 and the left-end reflector grating 7 are used to reflect the surface acoustic waves propagating to their respective locations, so that the surface acoustic waves form a stable round-trip propagation path inside the device, thereby improving the signal strength and frequency response stability of the device.

[0033] The interdigital transducer 2, the right-end reflector grating 3, and the left-end reflector grating 7 can adopt a composite metal layer structure consisting of a high-temperature resistant metal layer and an adhesive layer metal. The high-temperature resistant metal layer can be selected from one or more of platinum, tungsten, and molybdenum, while the adhesive layer metal can be selected from one or more of titanium, tantalum, and chromium. The high-temperature resistant metal layer ensures the conductivity stability of the electrode under high-temperature conditions, and the adhesive layer metal is located between the high-temperature resistant metal layer and the AlN transition layer 4 to enhance the adhesion strength between the electrode structure and the AlN transition layer 4. Preferably, the right-end reflector grating 3, the left-end reflector grating 7, and the interdigital transducer 2 use the same or similar metal material system to improve the acoustic consistency and structural stability of the device under thermal cycling conditions. The thickness range of the adhesive layer metal is preferably 10-30 nm. The thickness range of the high-temperature resistant metal layer is preferably 160-200 nm.

[0034] The upper alumina protective layer 1 covers the interdigital transducer 2, the right-end reflector grating 3, the left-end reflector grating 7, and the surface acoustic wave (SAW) propagation area. The upper alumina protective layer 1 serves to isolate the electrode structure and the SAW functional propagation area from the external environment, reducing the impact of high-temperature oxidation, corrosive media, and particle erosion on the electrodes and piezoelectric functional layer. The upper alumina protective layer 1 can be formed by magnetron sputtering, atomic layer deposition, chemical vapor deposition, physical vapor deposition, or other thin film deposition methods to obtain a dense, continuous, and stably adhered protective film.

[0035] In one alternative embodiment, the upper alumina protective layer 1 has a windowed area corresponding to the electrode pads, lead terminals, or antenna connection terminals to ensure electrical connection, signal readout, or wireless coupling of the device. This windowed area can be formed using patterned masks, etching, or selective deposition. With this structure, the upper alumina protective layer 1 can cover and protect the surface acoustic wave functional area while avoiding obstruction of necessary electrical connection or signal coupling areas.

[0036] In a preferred embodiment, the thickness of the upper alumina protective layer 1 is determined based on the surface acoustic wave (SAW) wavelength (i.e., the designed acoustic wavelength λ of the SAW along the propagation direction at the target operating frequency. For interdigital transducer structures, λ can be determined by the period of the interdigital electrodes; specifically, the center-to-center distance between adjacent interdigital electrodes of the same polarity corresponds to one SAW wavelength, and the center-to-center distance between adjacent interdigital electrodes of opposite polarity corresponds to half a SAW wavelength), the device operating frequency, and environmental protection requirements. To reduce excessive mass loading and elastic loading of the protective layer on SAW propagation, the thickness of the upper alumina protective layer 1 is preferably less than or equal to 0.3 times the SAW wavelength, for example, 0.05 to 0.3 times; preferably 0.1 to 0.3 times; if the thickness is greater than 0.3 times, it may affect the transmission of the upper surface wave; if it is less than 0.05 times, the protective strength is insufficient. It should be understood that this thickness relationship is used to illustrate a preferred design principle and does not limit the implementation of the invention in other thickness ranges.

[0037] In this embodiment, the upper alumina protective layer 1 exerts a certain mass and elastic loading on the propagation of surface acoustic waves (SAWs), which may cause a shift in the initial resonant frequency, a change in echo intensity, or a change in thermal response time. However, this effect is part of the fixed structural parameters of the device and can be compensated for through structural design and temperature-frequency calibration. The sensor in this embodiment is used for temperature detection, rather than relying on the detection of gases or humidity adsorbed by external molecules. Therefore, the upper alumina protective layer 1 does not block the measured temperature signal. The external temperature can be transferred to the interdigital transducer 2, the right-end reflective grating 3, the left-end reflective grating 7, the AlN transition layer 4, and the lithium niobate piezoelectric layer 5 through the thermal conduction of the upper alumina protective layer 1, causing the SAW propagation velocity or resonant frequency to shift with temperature changes, thereby achieving temperature detection.

[0038] Specifically, when the external temperature changes, the material parameters of the lithium niobate piezoelectric layer 5, the surface acoustic wave propagation velocity, and the device resonant frequency change accordingly. By reading the frequency signal output by the interdigital transducer 2 and establishing the correspondence between the resonant frequency and temperature, the external temperature can be detected. The initial frequency shift caused by the upper alumina protective layer 1 can be treated as an initial structural bias during the calibration process and does not affect the basic principle of the device's temperature detection based on frequency changes. Figure 4 As shown, Figure 4This is a schematic diagram illustrating the temperature-frequency response of the surface acoustic wave (SAW) temperature sensor of the present invention. As the external temperature changes, the propagation velocity of the SAW in the lithium niobate piezoelectric layer 5 and the resonant frequency of the device change accordingly. By pre-establishing the correspondence between the resonant frequency and temperature, the detection of the external temperature can be achieved. Figure 4 This invention is used to illustrate the temperature and frequency response principle of the sensor and is not intended to limit specific temperature ranges, sensitivity, or linearity.

[0039] In this embodiment, the fabrication process of the lithium niobate-based surface acoustic wave temperature sensor may include the following steps:

[0040] First, the substrate is cleaned and prepared. A lower alumina support protective layer 6 with a thickness of 300-600 μm and a single-crystal lithium niobate piezoelectric layer 5 with a thickness of 300-500 μm are provided. The lower alumina support protective layer 6 can be a polished alumina ceramic sheet, a sapphire sheet, or other alumina-based support sheet. The surfaces of the lower alumina support protective layer 6 and the single-crystal lithium niobate piezoelectric layer 5 to be bonded are cleaned, dried, and surface activated to remove surface contaminants and improve the subsequent bonding quality.

[0041] Next, wafer bonding or attachment is performed. The single-crystal lithium niobate piezoelectric layer 5 is disposed, bonded, or attached to the lower alumina support protective layer 6 to form a composite substrate structure consisting of the lower alumina support protective layer 6 and the single-crystal lithium niobate piezoelectric layer 5.

[0042] Next, an AlN transition layer 4, namely an aluminum nitride transition layer 4, is formed on the upper surface of the single-crystal lithium niobate piezoelectric layer 5. The thickness of the AlN transition layer 4 is 80-120 nm. The AlN transition layer 4 can be formed by reactive magnetron sputtering, magnetron sputtering, atomic layer deposition, chemical vapor deposition, or physical vapor deposition. It is used to improve the interfacial bonding between the subsequent electrode structure and the single-crystal lithium niobate piezoelectric layer 5, and reduce the risk of interface degradation or electrode desorption under high-temperature thermal cycling conditions.

[0043] Subsequently, photoresist is coated on the AlN transition layer 4, and patterned windows of the interdigital transducer 2, the right-end reflective grating 3, and the left-end reflective grating 7 are formed by exposure and development.

[0044] Then, an adhesion metal layer and a high-temperature resistant metal layer are sequentially deposited on the patterned structural surface. The thickness of the adhesion metal layer is 10-30 nm, and the thickness of the high-temperature resistant metal layer is 160-200 nm. The adhesion metal layer can be one or more of titanium, tantalum, and chromium, and the high-temperature resistant metal layer can be one or more of gold, platinum, tungsten, and molybdenum. After deposition, excess metal and photoresist are removed by a lift-off process, thereby forming an interdigitated transducer 2, a right-end reflective grating 3, and a left-end reflective grating 7 on the surface of the AlN transition layer 4.

[0045] Next, an upper alumina protective layer 1 is formed on the interdigital transducer 2, the right-end reflective grating 3, the left-end reflective grating 7, and the surface acoustic wave propagation region. The thickness of the upper alumina protective layer 1 is 150-250 nm. The upper alumina protective layer 1 can be formed by atomic layer deposition, magnetron sputtering, chemical vapor deposition, or physical vapor deposition to obtain a dense, continuous, and stably adhered protective film.

[0046] Subsequently, by using patterned masks, selective deposition, or post-deposition etching, windowed areas are formed at corresponding positions on electrode pads, lead ends, or antenna connection ends to expose the electrode pads, lead ends, or antenna connection ends for subsequent lead connection, antenna connection, or wireless signal coupling.

[0047] Finally, the fabricated surface acoustic wave temperature sensor is cut, packaged, or connected to an antenna, and the correspondence between the resonant frequency and temperature is established through temperature-frequency calibration to achieve the detection of the external temperature.

[0048] Through the above structure and preparation method, the present invention forms an aluminum oxide protection system on the upper and lower sides of the lithium niobate piezoelectric layer 5, introduces an AlN transition layer 4 between the lithium niobate piezoelectric layer 5 and the electrode structure, and adopts a high-temperature resistant composite metal electrode structure. In this way, while maintaining the functions of temperature signal transmission and surface acoustic wave frequency response, the device's structural stability, interface reliability and long-term working adaptability in high-temperature complex environments are improved.

[0049] This invention reduces the risk of thermal mismatch stress and interface failure of devices during temperature changes, corrosive environments, or long-term operation by matching the structural and material systems such as piezoelectric layers, support layers, interdigital transducers, and reflective gratings, thereby improving the structural stability, operational reliability, and environmental adaptability of the devices.

[0050] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, adjustments or substitutions can be made to the materials, thicknesses, fabrication processes, interdigitated transducer parameters, reflective grating forms, window positions, and signal readout methods of each layer without departing from the concept of the present invention; all such adjustments or substitutions should be included within the scope of protection of the present invention.

Claims

1. A surface acoustic wave sensor based on lithium niobate, characterized in that: The sensor comprises, from bottom to top, a lower alumina protective layer, a lithium niobate piezoelectric layer, an AlN transition layer, an electrode structure, and an upper alumina protective layer. The electrode structure includes an interdigital transducer and a reflective grid located on both sides of the interdigital transducer. The AlN transition layer is disposed between the lithium niobate piezoelectric layer and the electrode structure. The upper alumina protective layer covers the interdigital transducer, the reflective grid, and the surface acoustic wave propagation area, and has windowed areas at corresponding positions on the electrode pads, lead ends, or antenna connection ends to form a multi-layered protective surface acoustic wave sensing structure for high-temperature ambient temperature monitoring.

2. The surface acoustic wave sensor based on lithium niobate according to claim 1, characterized in that: The upper alumina protective layer has windowed areas at positions corresponding to electrode pads, lead ends, or antenna connection ends to ensure electrical connection or wireless signal coupling.

3. The surface acoustic wave sensor based on lithium niobate according to claim 1, characterized in that: The thickness of the upper alumina protective layer is less than 0.3 times the wavelength of the surface acoustic wave.

4. The surface acoustic wave sensor based on lithium niobate according to claim 1, characterized in that: The AlN transition layer is formed by magnetron sputtering, atomic layer deposition, chemical vapor deposition or physical vapor deposition.

5. The surface acoustic wave sensor based on lithium niobate according to claim 1, characterized in that: The interdigitated transducer includes a high-temperature resistant metal layer and an adhesive metal layer. The high-temperature resistant metal layer is one or more of platinum, tungsten, and molybdenum, and the adhesive metal layer is one or more of titanium, tantalum, and chromium.

6. The surface acoustic wave sensor based on lithium niobate according to claim 1, characterized in that: The reflective grating and the interdigital transducer use the same or similar metal material system.

7. The surface acoustic wave sensor based on lithium niobate according to claim 1, characterized in that: The reflective gratings are respectively disposed on the left and right sides of the interdigital transducer, and together with the interdigital transducer, they form a surface acoustic wave resonant cavity.

8. The surface acoustic wave sensor based on lithium niobate according to any one of claims 1 to 7, characterized in that: The thicknesses of each layer are as follows: top alumina layer 150-250 nm, high-temperature resistant metal layer 160-200 nm, adhesive layer 10-30 nm, AlN layer 80-120 nm, lithium niobate piezoelectric layer 300-500 μm, and bottom alumina layer 300-600 μm.

9. The surface acoustic wave sensor based on lithium niobate according to any one of claims 1 to 7, characterized in that: The surface acoustic wave temperature sensor establishes a correspondence between the resonant frequency and temperature through temperature-frequency calibration to compensate for the initial frequency shift caused by the upper alumina protective layer.

10. The method for fabricating a surface acoustic wave temperature sensor based on lithium niobate according to any one of claims 1 to 9, characterized in that: Includes the following steps: A lower alumina support and protective layer is provided, and a single crystal lithium niobate piezoelectric layer is disposed, bonded, or attached to the lower alumina support and protective layer. An AlN transition layer is formed on the single-crystal lithium niobate piezoelectric layer; An interdigital transducer and a reflective grating located on both sides of the interdigital transducer are formed on the AlN transition layer; An upper aluminum oxide protective layer is formed on the interdigital transducer, the reflective grating, and the surface acoustic wave propagation area; A window area is formed or retained at the corresponding position of the electrode pad, lead end, or antenna connection end; The AlN transition layer is formed by magnetron sputtering, reactive magnetron sputtering, atomic layer deposition, chemical vapor deposition, or physical vapor deposition; the upper alumina protective layer is formed by atomic layer deposition, magnetron sputtering, chemical vapor deposition, or physical vapor deposition; the interdigitated transducer and reflective grating are formed by photolithography, metal deposition, and lift-off processes, or by metal deposition, photolithography, and etching processes; and an adhesion metal layer is first formed on the AlN transition layer, and then a high-temperature resistant metal layer is formed on the adhesion metal layer.

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