High-stability surface acoustic wave strain sensor and preparation method thereof

Abstract

According to the high-stability surface acoustic wave strain sensor and the preparation method thereof, an electrode lateral protection process is improved, a silane coupling agent interface modification layer is introduced, the double-glue binding force is greatly improved, lateral protection failure caused by corrosive liquid permeation is avoided, meanwhile, a piezoelectric substrate is newly added for precise pretreatment, and the surface acoustic wave strain sensor has high stability. The roughness of the substrate is reduced through ultrasonic gradient cleaning, plasma activation and nanometer polishing, so that the electrode binding force and the sound wave conduction efficiency are improved.

Description

Technical Field

[0001] This invention belongs to the field of surface acoustic wave sensor technology, and relates to a high-stability surface acoustic wave strain sensor and its preparation method. Background Technology

[0002] In fields such as precision structures in aerospace, health monitoring, deformation feedback in micromechanical systems, and safety assessment of automotive battery packs, the stability and accuracy of strain measurement directly determine the engineering safety boundary. Traditional resistance strain gauges are susceptible to electromagnetic interference and have limited lifespan, while fiber optic sensors are limited in size and difficult to miniaturize. Surface acoustic wave (SAW) strain sensors have become an ideal solution due to their passive, wireless nature, high sensitivity, and environmental robustness. However, their core performance indicators, such as the quality factor, are easily destabilized by uneven stress distribution and sound wave scattering during dynamic deformation, leading to resonant frequency drift and a decrease in signal-to-noise ratio, which severely restricts the application of high-precision long-term monitoring.

[0003] Current surface acoustic wave strain sensors mainly detect strain by changing the sound wave propagation path caused by the deformation of the interdigital transducer substrate. However, they suffer from the problem of deformation sensitivity of the quality factor, that is, the strain gradient causes non-uniform dissipation of sound wave energy, and the quality factor fluctuation can reach more than 30%.

[0004] Therefore, there is an urgent need for a method to overcome the problem of deformation sensitivity of the quality factor in existing surface acoustic wave strain sensors. Summary of the Invention

[0005] To address the problems existing in the background technology, this invention proposes a high-stability surface acoustic wave strain sensor and its fabrication method. The aim is to overcome the problem of deformation sensitivity of the quality factor of existing surface acoustic wave strain sensors by combining electrode lateral protection technology and programmable constant-speed corrosion technology to achieve functional weighting of the interdigital electrode thickness, thereby enabling the strain sensor to maintain a stable quality factor across the entire range.

[0006] The first aspect of this application provides a method for fabricating a high-stability surface acoustic wave strain sensor, comprising: An electrode metal film is deposited on the pretreated piezoelectric substrate surface by magnetron sputtering. The electrode metal film includes a titanium transition layer and a gold functional layer. A process for lateral protection of the substrate after deposition of the electrode metal film is performed. The lateral protection process includes first adhesive coating and patterning, preparation of interface modification layer, second adhesive coating and removal of first adhesive. A multi-parameter closed-loop controlled constant-rate etching process is used to process the substrate after the electrode lateral protection process. The multi-parameter closed-loop controlled constant-rate etching process includes preparation of etching solution and equipment, closed-loop control of concentration during etching, variable speed control of spin coater rotation, and closed-loop control of temperature. The electrode surface of the etched substrate is finely repaired, and the repaired substrate is then subjected to a second adhesive removal and cleaning.

[0007] Optionally, the piezoelectric substrate pretreatment includes: The piezoelectric substrate was subjected to ultrasonic treatment in sequence with acetone solution, anhydrous ethanol, and deionized water. After ultrasonic treatment, the piezoelectric substrate is placed in a nitrogen drying oven for drying. After drying, the piezoelectric substrate is placed in a vacuum plasma treatment instrument for plasma activation modification; The piezoelectric substrate was then subjected to nanoscale chemical mechanical polishing after plasma activation modification. After nanoscale chemical mechanical polishing, the piezoelectric substrate was rinsed with deionized water and then dried with nitrogen gas.

[0008] Optionally, during the plasma activation and modification process, oxygen is introduced into the vacuum plasma processor.

[0009] Optionally, the titanium and gold targets are pre-sputtered before the electrode metal film is deposited by magnetron sputtering.

[0010] Optionally, during the first resist coating and patterning process, the first resist is selected as polyimide photoresist; the first resist is coated onto the surface of the electrode metal film by spin coating; after the first resist is coated, a pre-baking treatment is performed, and after the pre-baking, the first resist is exposed using an ultraviolet lithography machine through an interdigitated electrode mask; after exposure, the first resist is developed using a developer; after development, the first resist is rinsed with deionized water; after rinsing, the first resist is subjected to a hard baking treatment.

[0011] Optionally, the material of the interface modification layer is a silane coupling agent; during the vapor deposition process, the vacuum degree is not higher than 1 x 10^-3 Pa; and the thickness of the interface modification layer is controlled between 10 nanometers and 20 nanometers.

[0012] Optionally, during the second resist coating and first resist removal process, the second resist is a nanoimprint photoresist; the second resist is coated by spin coating; after the second resist is coated, a post-baking treatment is performed; during the first resist removal, the substrate is immersed in an N-methylpyrrolidone solution; and ultrasonic assistance is used during the first resist removal process.

[0013] Optionally, during the corrosion process, the concentration closed-loop control is achieved by using a UV-Vis spectrophotometer to measure the absorbance of the corrosive solution in real time. Calculate the concentration of the etching solution according to the Lambert-Beer law. The expression of the Lambert-Beer law is A(λ)=ε(λ)×C(t)×L, where A(λ) is the absorbance of the etching solution, ε(λ) is the molar absorption coefficient of the etching solution, C(t) is the molar concentration of the etching solution at time t, and L is the optical path length. When the calculated C(t) is less than 90% of the initial concentration, add the etching solution mother liquor through the second micro-syringe in the programmed dual micro-syringe. The initial concentration is calibrated by an inductively coupled plasma emission spectrometer.

[0014] Optionally, the variable-speed rotation control of the spin coater adopts a three-segment speed curve. The expression of the three-segment speed curve is: when 0≤t≤t1, n(t)=n1+(n2 - n1)×(t / t1); when t1<t≤t2, n(t)=n2; when t2<t≤t 总 when, n(t)=n2-(n2 - n1)×((t - t2) / (t 总 - t2)), where n(t) is the rotation speed of the spin coater at time t, n1 is the initial rotation speed of the spin coater, n2 is the maximum rotation speed of the spin coater, t 总 is the total etching time, t1 is the acceleration time, and t2 is the end time of the uniform speed.

[0015] In the second aspect of the present application, a high-stability surface acoustic wave strain sensor is provided. The surface acoustic wave strain sensor includes: a weighted interdigital electrode, a piezoelectric substrate, and a packaging shell.

[0016] Compared with the prior art, the present invention has the following beneficial effects: In the electrode side protection process of the present invention, a silane coupling agent interface modification layer is introduced, and the double-bonding force is greatly improved, avoiding the side protection failure caused by the penetration of the etching solution. At the same time, a new precision pretreatment of the piezoelectric substrate is added. By plasma activation and nano-polishing, the substrate roughness is reduced to improve the electrode bonding force and the acoustic wave conduction efficiency. Description of the Drawings

[0017] Figure 1 is a flowchart of a preparation method of a high-stability surface acoustic wave strain sensor according to an embodiment of the present invention; Figure 2 is a schematic diagram of a high-stability surface acoustic wave strain sensor according to an embodiment of the present invention.

[0018] Note: 1-1, weighted interdigital electrode; 1-2, piezoelectric substrate; 1-3, packaging shell. Detailed Embodiments

[0019] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0020] In one embodiment, such as Figure 1 As shown, a method for fabricating a highly stable surface acoustic wave strain sensor is provided, which is then applied to... Figure 1 Taking China as an example, the following specific steps will be used: S10: Electrode metal films are deposited on the pretreated piezoelectric substrate surface using magnetron sputtering.

[0021] Specifically, in this invention, the precision pretreatment of the piezoelectric substrate is a key step that achieves ultra-cleanliness and low roughness of the substrate surface through multi-stage processing, laying the foundation for the uniform deposition of the subsequent electrode metal film and the stable propagation of acoustic waves. This process specifically includes the following: Ultrasonic gradient cleaning: used to remove oil, organic residues, and particulate impurities from the surface of piezoelectric substrates. First, the piezoelectric substrate is immersed in an acetone solution and ultrasonically treated for 10 minutes at 300 watts and 40 degrees Celsius, utilizing the dissolving effect of acetone to remove surface oil. Then, the substrate is transferred to anhydrous ethanol and ultrasonically treated for 10 minutes at 300 watts and 35 degrees Celsius to remove residual acetone. Finally, the substrate is placed in an environment with a resistivity of not less than 18.2 megohms. In centimeters of deionized water, ultrasonic treatment at 250 watts and 30 degrees Celsius for 15 minutes completely removes organic residues.

[0022] Nitrogen drying: After ultrasonic gradient cleaning, the substrate should be placed in a nitrogen drying oven immediately and dried for 20 minutes at a nitrogen flow rate of 5 liters / minute and a temperature of 50 degrees Celsius to avoid water stains or secondary contamination on the substrate surface due to natural air drying.

[0023] Plasma activation modification: The dried substrate is placed in a vacuum plasma treatment instrument, and oxygen with a purity of not less than 99.99% is introduced into the environment with a vacuum degree not exceeding 5 Pa. The power is set to 150 watts, the treatment time to 2 minutes, and the gas flow rate to 20 standard cubic centimeters per minute. Through plasma bombardment, trace carbon contaminants on the substrate surface can be removed, while hydroxyl groups are introduced into the surface, reducing the surface water contact angle to below 20 degrees, significantly improving the wettability and adhesion of subsequent photoresists.

[0024] Nanoscale chemical mechanical polishing: The substrate is polished using a colloidal silica polishing slurry with a particle size of 50 nanometers and a concentration of 20% by weight on a precision polishing machine. The polishing pressure is set to 0.1 MPa, the polishing pad speed is 300 rpm, the substrate speed is 350 rpm, and the polishing time is 5 minutes. Through the synergistic effect of mechanical abrasion and chemical etching, microscopic scratches on the substrate surface are eliminated.

[0025] Rinsing and Drying: After polishing, the substrate is rinsed with deionized water for 5 minutes to remove any residual polishing solution. It is then dried with nitrogen gas to ensure no liquid residue remains on the substrate surface. Atomic force microscopy analysis shows that this pretreatment reduces the substrate surface roughness to no more than 2 nanometers.

[0026] Electrode metal films were deposited on the pretreated piezoelectric substrate surface using magnetron sputtering. A composite structure of a titanium transition layer and a gold functional layer was employed to balance the adhesion between the electrode and the substrate with the electrode's conductivity. The specific process is as follows: A titanium target with a purity of 99.95% and a gold target with a purity of 99.99% were selected. The titanium target was used to prepare the transition layer to improve the adhesion between the gold functional layer and the substrate, while the gold target was used to prepare the functional layer to ensure low resistance and high acoustic wave transmission efficiency. Before sputtering, the titanium and gold targets were pre-sputtered at a power of 200 watts for 10 minutes to remove the oxide layer on the target surface and ensure the purity of the deposited film.

[0027] The pretreated piezoelectric substrate was placed in the vacuum chamber of the magnetron sputtering equipment and evacuated to a vacuum level not exceeding 3 x 10⁻⁴ Pa. Argon gas with a purity of not less than 99.999% was introduced as the working gas, with a flow rate of 30 standard cubic centimeters per minute. When depositing the titanium transition layer, the sputtering power was set to 150 W and the deposition rate to 0.5 nm / s. The thickness was monitored in real time using a quartz crystal oscillator to ensure that the transition layer thickness was between 50 nm and 100 nm. When depositing the gold functional layer, the sputtering power was set to 250 W and the deposition rate to 1 nm / s. At the same time, the substrate temperature was controlled at 80 degrees Celsius to reduce intrafilm stress and ensure that the functional layer thickness was between 300 nm and 500 nm.

[0028] After the electrode metal film is deposited, the substrate is annealed in a nitrogen atmosphere at a temperature of 150 degrees Celsius for 30 minutes. This process slowly releases the internal stress generated during sputtering, preventing the electrode metal film from cracking or peeling off in subsequent processes.

[0029] S20: Electrode lateral protection process for the substrate after deposition of electrode metal film.

[0030] Specifically, the electrode lateral protection process achieves precise separation between surface exposure and side protection of the interdigitated electrode through the synergistic effect of the first adhesive patterning and shaping, the interface modification layer strengthening the bond, and the second adhesive lateral sealing, thus avoiding excessive lateral corrosion of the electrode in subsequent corrosion processes. It includes three sub-steps: first adhesive coating and patterning, interface modification layer preparation, and second adhesive coating and first adhesive removal.

[0031] The purpose of the first adhesive coating and patterning is to form a preliminary pattern of interdigitated electrodes on the surface of the electrode metal film, providing a basis for the subsequent positioning of the interface modification layer and the second adhesive. The specific operation process and parameter control are as follows: First resist selection and coating: Polyimide photoresist was selected as the first resist due to its high temperature resistance and chemical corrosion resistance, making it suitable for subsequent etching environments. The first resist was applied to the piezoelectric substrate surface with the deposited electrode metal film using spin coating. The spin coating speed was set to 3000 rpm, and the spin coating time was 30 seconds to ensure a uniform resist layer on the electrode metal film surface, with a thickness controlled between 1.5 and 2 micrometers, free from defects such as bubbles and pinholes.

[0032] Pre-baking treatment: The substrate coated with the first adhesive is placed in a hot air oven for segmented pre-baking to gradually remove the solvent in the adhesive layer and avoid cracking or bubbling caused by direct high-temperature baking. The pre-baking process strictly follows the segmented settings of "low temperature, medium temperature, high temperature": first, maintain at 60 degrees Celsius for 30 minutes, then raise the temperature to 90 degrees Celsius and maintain for 30 minutes, and finally raise the temperature to 120 degrees Celsius and maintain for 60 minutes; during the pre-baking process, air circulation must be maintained in the oven to prevent solvent volatiles from condensing on the adhesive layer surface.

[0033] Patterned Exposure: The pre-baked substrate is placed in a UV lithography machine and exposed using an interdigitated electrode mask as a template. The interdigitated electrode mask has a linewidth of 5 to 10 micrometers and is consistent with the designed electrode pattern. The UV lithography machine is set to an exposure wavelength of 365 nanometers and an exposure dose of 150 millijoules per square centimeter to ensure that the exposed area of ​​the first resist undergoes a photochemical reaction, preparing for subsequent development and shaping. During exposure, precise alignment between the substrate and the mask must be ensured, with alignment errors controlled within 0.1 micrometers to avoid electrode pattern misalignment.

[0034] Development and Hard Baking: Immediately after exposure, the substrate is immersed in a developer solution specifically for polyimide photoresist, with a development time controlled at 2 minutes. The developer dissolves the unexposed areas of the first resist, exposing the underlying electrode metal film and forming an interdigitated electrode pattern consistent with the photomask. After development, the substrate surface is rinsed with deionized water for 2 minutes until the rinsing solution is neutral (pH 7). Finally, the substrate is placed in a hot air oven for hard baking at 150 degrees Celsius for 60 minutes. This high-temperature curing further enhances the structural strength and solvent resistance of the first resist pattern.

[0035] The purpose of preparing the interface modification layer is to enhance the bonding force between the first adhesive and the subsequent second adhesive, while forming a hydrophobic barrier to prevent the subsequent etching solution from penetrating to the electrode side through the gap between the two adhesives. The specific operation is as follows: Material selection and deposition equipment: A silane coupling agent (such as KH-550) was selected as the interface modification layer material. The amino group (-N) at one end of the molecule of this material... It can form a covalent bond with the carboxyl group (-COOH) on the surface of the first adhesive, and the alkoxy group (-OC) at the other end... The silane coupling agent can combine with the hydroxyl groups (-OH) on the surface of the second adhesive to achieve chemical anchoring of the two adhesive interfaces; at the same time, the film formed after the silane coupling agent is cured is hydrophobic and can block the penetration path of the corrosive liquid. The interface modification layer was prepared by vapor phase deposition, and a vacuum vapor phase deposition instrument was used as the deposition equipment.

[0036] Deposition parameter control: The substrate with the first gel patterned is placed into the chamber of a vacuum vapor deposition apparatus. The chamber is closed and a vacuum is drawn, reducing the vacuum level to no more than 1 x 10^-3 Pascals. The silane coupling agent is then heated to 80 degrees Celsius to vaporize and introduced into the chamber for deposition. The deposition time is controlled at 5 minutes. The deposition thickness is monitored in real time using a quartz crystal oscillator to ensure that the thickness of the interface modification layer is controlled between 10 and 20 nanometers. Too thin a layer will result in insufficient adhesion, while too thick a layer may cover the exposed area of ​​the electrode metal film.

[0037] Curing of the modified layer: After deposition, the vacuum level in the chamber is kept constant, and the temperature is maintained at 80 degrees Celsius for 10 minutes to allow the silane coupling agent molecules to fully react with and cure the surface of the first adhesive, forming a stable interface modified layer. Then, nitrogen gas is slowly introduced into the chamber with a purity of not less than 99.99% until the pressure in the chamber is balanced with the atmospheric pressure. The substrate is then removed to avoid sudden pressure changes that could cause the modified layer to fall off.

[0038] The application of the second adhesive and the removal of the first adhesive are key steps in achieving electrode surface exposure and side protection. By applying a second adhesive with a different adhesive remover than the first adhesive, the second adhesive maintains the seal on the electrode sides while the first adhesive is removed. The specific operation is as follows: Second resist selection and coating: Nanoimprint photoresist was selected as the second resist. The remover for this material is isopropanol, which is immiscible with N-methylpyrrolidone, the remover for the first resist, thus preventing the second resist from dissolving during the removal process. The second resist was spin-coated onto the substrate surface of the prepared interface-modified layer. The spin-coating speed was set to 2500 rpm, and the spin-coating time was 30 seconds, ensuring that the second resist completely covered the pattern of the first resist, the interface-modified layer, and the non-patterned areas of the substrate, forming a uniform resist layer with a thickness of 1 to 1.2 micrometers.

[0039] Post-bake of the second adhesive: The substrate coated with the second adhesive is placed in a hot air oven for post-bake to cure the second adhesive and improve its solvent resistance. The post-bake process is set in stages: first, it is kept at 80 degrees Celsius for 30 minutes, and then the temperature is increased to 110 degrees Celsius and kept for 30 minutes; during the post-bake process, the heating rate must be controlled at 5 degrees Celsius per minute to avoid stress cracking of the adhesive layer due to excessive temperature difference.

[0040] First adhesive removal: The post-baked substrate was immersed in an N-methylpyrrolidone solution, and the first adhesive was removed by solvent immersion combined with ultrasonic assistance. The temperature of the N-methylpyrrolidone solution was set to 60 degrees Celsius, the ultrasonic power was 100 watts, and the ultrasonic time was 30 minutes. High temperature and ultrasound can accelerate the dissolution of the first adhesive by N-methylpyrrolidone without affecting the second adhesive. The second adhesive is insoluble in N-methylpyrrolidone. After the first adhesive is completely dissolved, the substrate was removed and rinsed with deionized water to remove any remaining N-methylpyrrolidone solution for 5 minutes, followed by drying the substrate surface with nitrogen. At this point, the electrode metal film surface originally covered by the first adhesive was completely exposed, while the electrode sides and non-patterned areas of the substrate were tightly wrapped by the second adhesive, achieving lateral protection of the electrodes.

[0041] S30: A programmable constant-rate etching process with multi-parameter closed-loop control for substrates that have undergone electrode lateral protection.

[0042] Specifically, the core of the multi-parameter closed-loop controlled constant-speed etching process is to achieve precise control of the interdigitated electrode thickness by real-time monitoring and dynamic adjustment of the etching solution concentration, spin coater speed, and substrate temperature. This avoids uneven etching caused by fluctuations in a single parameter and ensures that the weighted depth of the electrode in the strain concentration area meets the design requirements. Specifically, it includes four sub-steps: etching solution and equipment preparation, concentration closed-loop control during etching, spin coater speed control, and temperature closed-loop control.

[0043] The preparation of the etching solution and equipment is fundamental to ensuring the feasibility of subsequent closed-loop control. It is necessary to clearly define the etching solution formula, equipment composition, and substrate positioning requirements. Specific procedures are as follows: Preparation and calibration of the etching solution: A gold etching solution was selected as the etching medium, and its formula was determined by volume ratio as concentrated nitric acid: concentrated hydrochloric acid: deionized water = 1:3:6. After preparation, the initial concentration of the etching solution was calibrated using inductively coupled plasma atomic emission spectrometry (ICP-AES) to ensure that the initial concentration C0 was stable at 0.5 mol / L. This concentration balances etching efficiency and etching uniformity, avoiding excessive corrosion of the electrode due to excessive concentration or prolonged process time due to excessively low concentration. Simultaneously, a mother liquor of the etching solution (concentration of 2 mol / L) was prepared for subsequent concentration compensation.

[0044] Equipment Combination and Debugging: The system employs a combination of a programmable dual micro-syringe, an intelligent spin coater, a UV-Vis spectrophotometer, and an infrared thermometer. Specifically: the programmable dual micro-syringe has two independent injection channels with a range of 1-100 μL and an accuracy of 0.1 μL, used for delivering the etching solution and the mother liquor respectively. The intelligent spin coater, with a speed range of 50-500 rpm and an accuracy of 1 rpm, is used to support the substrate and achieve variable speed rotation. The UV-Vis spectrophotometer, with a detection wavelength range of 300-800 nm and an accuracy of 0.001 absorbance units, is used for real-time measurement of the absorbance of the etching solution. The infrared thermometer, with a measurement range of 0-100 degrees Celsius and an accuracy of ±0.1 degrees Celsius, is used for real-time monitoring of the substrate surface temperature. During equipment debugging, it is necessary to ensure that the injection speed of the programmable dual micro-syringe and the rotation speed response delay of the intelligent spin coater do not exceed 0.5 seconds, the detection optical path of the UV-Vis spectrophotometer is aligned with the contact area of ​​the etching solution, and the detection point of the infrared thermometer is directly facing the center area of ​​the substrate.

[0045] Substrate fixation and alignment: The substrate with the completed electrode lateral protection process is fixed on the stage of the intelligent spin coater. The position of the substrate is adjusted by the optical alignment system so that the center area of ​​the interdigital electrode is coaxially aligned with the injection outlet of the programmable dual micro-syringe. The alignment error is controlled within 0.1 mm. This alignment accuracy can ensure that the etching solution acts preferentially on the strain concentration area that needs to be weighted, and avoid the weighting position deviation caused by the offset of the etching area.

[0046] The purpose of closed-loop concentration control during corrosion is to address the concentration decay of the corrosion solution caused by reaction consumption. Concentration is calculated by real-time absorbance monitoring and the mother liquor is replenished to maintain a stable concentration. The specific operation is as follows: Real-time absorbance monitoring: During the corrosion process, the UV-Vis spectrophotometer continuously monitors the absorbance A(λ) of the corrosion solution at a characteristic wavelength of 450 nm, with a monitoring frequency of 1 time / second. 450 nm was chosen as the characteristic wavelength because the absorption intensity of gold ions in the corrosion solution at this wavelength is linearly related to the concentration, which can accurately reflect the concentration change.

[0047] Concentration calculation and judgment: Calculate the real-time etching solution concentration C(t) according to the Lambert-Beer law, and the expression of this law is A(λ)=ε(λ)×C(t)×L, where: A(λ) is the real-time absorbance of the etching solution at a wavelength of 450 nm (measured by an ultraviolet-visible spectrophotometer); ε(λ) is the molar absorptivity of the etching solution at a wavelength of 450 nm; C(t) is the molar concentration of the etching solution at time t (the value to be calculated); L is the optical path length. Each time C(t) is calculated, it is compared with 90% of the initial concentration, that is, 0.45 mol / L, to determine whether the mother liquor needs to be replenished.

[0048] Mother liquor replenishment control: When the calculated C(t) is less than 0.45 mol / L, the program control system immediately triggers the mother liquor channel of the program-controlled double micro-syringe, and calculates the replenishment volume according to the formula V 补 =(C0×V 总 -C(t)×V 总 ) / C 母 , where V 补 is the mother liquor replenishment volume, V 总 is the total volume of the current etching solution, C0 is the initial concentration, C 母 is the mother liquor concentration, and the mother liquor is replenished at a speed of 0.5 μL / s until C(t) returns to the range of 0.5 mol / L ± 5%, and then the mother liquor channel is closed. The replenishment method can avoid sudden increases and decreases in concentration and ensure a stable etching rate.

[0049] The purpose of the variable-speed rotation control of the spin coater is to enable the etching solution to spread evenly on the substrate surface, react stably, and avoid excessive etching at the edges through a three-stage speed curve. The specific operations are as follows: Speed curve design: Adopt an acceleration, constant speed, and deceleration three-stage speed curve, and the curve expression is divided according to the time stage: Acceleration stage (0≤t≤t1): n(t)=n1+(n2 - n1)×(t / t1), where n(t) is the speed of the spin coater at time t, n1 is the initial speed, n2 is the maximum speed, and t1 is the acceleration time; in the acceleration stage, the speed is gradually increased to enable the just-injected etching solution to evenly cover the substrate surface under the action of centrifugal force and avoid local liquid accumulation.

[0050] Constant speed stage (t1<t≤t2): n(t)=n2, where t2 is the end time of the constant speed, and t2=t 总 -10 s, t 总 is the total etching time; in the constant speed stage, the constant speed is used to maintain the uniform distribution of the etching solution on the substrate surface, ensure the consistency of the etching reaction rate, and avoid local etching depth differences caused by speed changes.

[0051] Deceleration stage (t2<t≤t 总): n(t) = n2 - (n2 - n1) × ((t - t2) / (t 总 - t2)); In the deceleration stage, the rotational speed gradually decreases to reduce the pulling effect of centrifugal force on the edge of the electrode in the later stage of corrosion, avoiding the corrosion liquid becoming too thin in the edge area due to excessive centrifugal force, and thus preventing excessive corrosion.

[0052] Real-time adjustment of rotational speed: The control system of the intelligent spin coater receives the rotational speed instruction sent by the program control system in real time, and the response time for rotational speed adjustment does not exceed 0.2 seconds; at the same time, the actual rotational speed is real-time fed back through the rotational speed sensor on the spin coater pedestal. If the deviation between the actual rotational speed and the command rotational speed exceeds 2 revolutions per minute, fine adjustment is immediately triggered to ensure that the rotational speed accuracy meets the requirements.

[0053] The purpose of temperature closed-loop control is to suppress the temperature rise caused by the heat release of the corrosion reaction and avoid the influence of temperature fluctuations on the corrosion rate. The specific operations are as follows: Real-time temperature monitoring: The infrared thermometer continuously monitors the real-time temperature T(t) of the substrate surface. The monitoring point is fixed at the central area of the substrate, which coincides with the core area of the corrosion liquid action, and the monitoring frequency is 1 time per second. The data is transmitted to the program control system in real time.

[0054] Temperature judgment and regulation: Set the target temperature T0 to 25 degrees Celsius, and regulate according to the deviation between T(t) and T0: When T(t) > T0 + 0.5 degrees Celsius, the program control system activates the micro cooling fan built into the spin coater pedestal. The fan speed is adjusted in grades according to the deviation. That is, when the deviation is 0.5 - 1 degree Celsius, the wind speed is 3 m / s, and when the deviation > 1 degree Celsius, the wind speed is 5 m / s; at the same time, if T(t) still does not decrease within 10 seconds after the wind speed is adjusted, the rotational speed of the spin coater is appropriately increased, with a maximum of no more than 250 revolutions per minute, to accelerate heat dissipation by enhancing air flow until T(t) drops back within the range of 25 degrees Celsius ± 0.5 degrees Celsius.

[0055] When T(t) < T0 - 0.5 degrees Celsius, the program control system activates the heating module built into the pedestal, that is, with a power of 5 - 10 watts, and adjusts in grades according to the deviation: when the deviation is 0.5 - 1 degree Celsius, the power is 5 watts, and when the deviation > 1 degree Celsius, the power is 10 watts, until T(t) rises back within the range of 25 degrees Celsius ± 0.5 degrees Celsius, and then the heating module is turned off. The above regulation method can avoid sudden temperature rises and drops and maintain the stability of the corrosion rate.

[0056] S40: Perform fine repair on the surface of the electrode of the corroded substrate, and remove the second layer of glue and clean the substrate after fine repair.

[0057] Specifically, the core of electrode surface finishing is to remove residual adhesive residue and corrosion products from the electrode surface after corrosion, reducing surface roughness to minimize acoustic wave scattering loss. Second adhesive removal and cleaning thoroughly remove the second adhesive used for lateral protection and any remaining surface impurities, laying a clean substrate foundation for subsequent encapsulation or performance testing. Both processes must be performed in the order of finishing first, followed by adhesive removal and cleaning.

[0058] The electrode surface finishing process employs low-temperature argon plasma treatment technology. This technology removes residual contaminants through the physical bombardment of plasma, while simultaneously preventing damage to the electrode's metal film from high temperatures. The specific operating procedures and parameter controls are as follows: Pre-treatment before finishing: Remove the substrate that has completed the multi-parameter closed-loop controlled constant-speed etching process from the intelligent spin coater stand. First, quickly rinse the surface with deionized water to remove any residual etching solution for 2 minutes. Then, blow the substrate surface dry with nitrogen to avoid the residual etching solution from reacting with argon gas in the subsequent plasma treatment or causing secondary corrosion of the electrodes.

[0059] A vacuum plasma treatment system was selected as the finishing equipment. The equipment must have cryogenic control capabilities, meaning the substrate temperature must not exceed 50 degrees Celsius during treatment. The pretreated substrate was placed in the equipment chamber, which was then closed and evacuated to stabilize the vacuum level at 5 Pa. Argon gas was then introduced as the working gas, with a flow rate of 15 standard cubic centimeters per minute to ensure a stable and impurity-free gas atmosphere within the chamber. The plasma treatment power was set to 80 watts, and the treatment time to 45 seconds. Insufficient power would result in incomplete removal of residual contaminants, while excessive power or prolonged treatment time could damage the gold functional layer. The cryogenic environment below 50 degrees Celsius prevented stress deformation of the electrode metal film due to high temperatures.

[0060] After starting the vacuum plasma processor, monitor the vacuum level and substrate temperature in the chamber in real time to ensure that the vacuum level is always maintained within the range of 5 Pa ± 0.5 Pa and the substrate temperature does not exceed 50 degrees Celsius. During the processing, the plasma is uniformly distributed in a light blue color without local spots or uneven brightness. If any abnormality occurs, stop the equipment immediately and check the parameter settings.

[0061] After processing, nitrogen gas is slowly introduced into the chamber until atmospheric pressure is reached, and the substrate is removed. The electrode surface is observed using a scanning electron microscope to ensure that there are no visible glue residues or corrosion products, and that the electrode surface is free of scratches or dents. At the same time, the surface roughness of the electrode is detected using an atomic force microscope, which must be controlled between 5 nanometers and 10 nanometers to meet the requirements of acoustic wave transmission for surface flatness.

[0062] The removal and cleaning of the second adhesive needs to be carried out in three sub-steps: dissolving and removing the second adhesive, cleaning the residual adhesive remover, and removing organic impurities to ensure thorough removal of the second adhesive and all kinds of residues. The specific operation is as follows: Second adhesive removal: Isopropanol solution was used as the adhesive remover for the second adhesive. The substrate, after surface finishing, was completely immersed in the isopropanol solution, with the temperature controlled at 40 degrees Celsius. 40 degrees Celsius accelerates the dissolution rate of the second adhesive while preventing excessive evaporation of the isopropanol due to high temperatures. The container soaking the substrate was placed in an ultrasonic cleaner, with the ultrasonic power set to 80 watts and the ultrasonic time set to 20 minutes. Ultrasonic vibration breaks down the adhesion between the second adhesive and the substrate and electrode sides, promoting the detachment of the dissolved second adhesive from the substrate surface. During the ultrasonic process, it is essential to ensure that the substrate is completely immersed in the isopropanol solution, with no localized exposure, to avoid areas where the adhesive removal is incomplete.

[0063] Deionized water cleaning: After removing the second adhesive, take the substrate out of the isopropanol solution and immediately put it into deionized water. Start the ultrasonic cleaner again, set the ultrasonic power to 80 watts and the ultrasonic time to 5 minutes. Deionized water cleaning can remove the residual isopropanol solution and dissolved second adhesive fragments on the substrate surface, and prevent the residual isopropanol from mixing and reacting with anhydrous ethanol in subsequent steps, or forming water stains on the substrate surface.

[0064] Anhydrous ethanol cleaning: After rinsing with deionized water, transfer the substrate to anhydrous ethanol and continue ultrasonic cleaning. Set the ultrasonic power to 80 watts and the ultrasonic time to 5 minutes. Anhydrous ethanol can dissolve residual organic impurities on the substrate surface, such as incompletely removed adhesive residue. It also has rapid evaporation properties, reducing the time required for subsequent drying steps. Freshly prepared anhydrous ethanol must be used during the cleaning process to avoid secondary adhesion of impurities due to decreased ethanol purity.

[0065] Nitrogen drying: After cleaning with anhydrous ethanol, immediately remove the substrate and slowly dry it with a nitrogen gun at a 45-degree angle along the substrate surface to avoid scratches caused by excessive airflow speed. The drying order is to dry the front of the substrate first, then the back, to ensure no liquid residue remains. After drying, place the substrate in a clean desiccator for later use to avoid recontamination by dust and impurities in the air.

[0066] In one embodiment, such as Figure 2 As shown, a high-stability surface acoustic wave (SAW) strain sensor is provided. This high-stability SAW strain sensor corresponds one-to-one with the fabrication method of a high-stability SAW strain sensor in the above embodiments. The high-stability SAW strain sensor includes: a weighted interdigitated electrode 1-1, a piezoelectric substrate 1-2, and a packaging shell 1-3. Detailed descriptions of each functional module are as follows: The weighted interdigitated electrode 1-1 is the core functional component of the sensor for achieving strain detection and quality factor stabilization. Its structural design revolves around suppressing acoustic energy dissipation caused by strain: the electrode is based on a composite structure of titanium transition layer and gold functional layer deposited on the surface of piezoelectric substrate. The key is to perform thickness function weighting on the central region where strain is concentrated. This central region needs to be determined according to the formula, and the weighting depth is controlled between 30% and 60% of the total electrode thickness. That is, weighting is required when the range is greater than 600uε, and the maximum depth of 60% is taken when the range is greater than 2000uε. Weighting can be achieved through linear functions, circular arc functions, etc., to ultimately ensure that the acoustic wave propagation of the sensor is stable throughout the entire range and avoid large fluctuations in the quality factor due to strain gradient.

[0067] The piezoelectric substrate 1-2 is the carrier for surface acoustic wave propagation. It needs to be made of materials with excellent piezoelectric properties, such as quartz, lithium niobate, lithium tantalate, or silicon wafers. It also needs to undergo precise pretreatment such as ultrasonic gradient cleaning, plasma activation modification, and nanoscale chemical mechanical polishing to ensure that the surface roughness does not exceed 2nm. This ensures a tight bond between the interdigital electrodes and the substrate and eliminates the interference of microscopic defects on the propagation of sound waves. At the same time, the length, width, and thickness parameters of the substrate need to be matched with the sensor range and weighting area design to provide a stable basis for the conversion of strain signals into surface acoustic wave signals.

[0068] The encapsulation shell 1-3 is a protective component that ensures the long-term stable operation of the sensor. It needs to completely enclose the weighted interdigitated electrodes and piezoelectric substrate to form a sealed protective space. Its design must balance environmental adaptability and performance interference-free operation: on the one hand, it must isolate external humidity, dust, corrosive gases, and mechanical impact to avoid oxidation of internal electrodes, damage to the substrate, or interference with the sound wave propagation path. Therefore, materials such as metals or ceramics, which combine high strength and sealing performance, are often selected. On the other hand, it must ensure that the encapsulation process does not change the stress state of the piezoelectric substrate, and that the connection method between the shell and the substrate does not affect the transmission of strain signals. That is, external strain can be accurately transmitted to the substrate through the encapsulation structure and then converted into changes in the frequency of surface acoustic waves. At the same time, it should avoid the deformation of the shell itself from causing additional interference to the sensor's detection accuracy. Ultimately, this enables the sensor to maintain long-term stable strain detection performance in harsh scenarios such as aerospace precision structure monitoring and automotive battery pack safety assessment.

[0069] Specific limitations regarding the high-stability surface acoustic wave strain sensor can be found in the above description of the fabrication method for such a sensor, and will not be repeated here. Each module in the high-stability surface acoustic wave strain sensor can be implemented entirely or partially through software, hardware, or a combination thereof. These modules can be embedded in hardware or independently of the processor in a computer device, or stored in software in the memory of a computer device, allowing the processor to call and execute the corresponding operations of each module.

[0070] Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for manufacturing a high-stability surface acoustic wave strain sensor, characterized by, The application relates to a preparation method of a piezoelectric substrate electrode, and belongs to the technical field of piezoelectric substrate electrode preparation. The electrode metal film is deposited on the surface of the pretreated piezoelectric substrate by a magnetron sputtering method, and the electrode metal film comprises a titanium transition layer and a gold functional layer; An electrode lateral protection process is carried out on the substrate after the electrode metal film is deposited, and the electrode lateral protection process comprises first glue coating and patterning, interface modification layer preparation, second glue coating and first glue removal; A multi-parameter closed-loop control program-controlled constant-speed etching process is carried out on the substrate after the electrode lateral protection process, and the multi-parameter closed-loop control program-controlled constant-speed etching process comprises etching liquid and equipment preparation, concentration closed-loop control during etching, variable-speed rotation control of a glue uniformizing machine and temperature closed-loop control; The electrode surface of the substrate after etching is refined, and the refined substrate is subjected to second glue removal and cleaning.

2. The production method according to claim 1, characterized by, The piezoelectric substrate pretreatment comprises the following steps: The piezoelectric substrate is sequentially subjected to ultrasonic treatment by using acetone solution, anhydrous ethanol and deionized water; After the ultrasonic treatment, the piezoelectric substrate is placed in a nitrogen drying box for drying; After drying, the piezoelectric substrate is placed in a vacuum plasma treatment instrument for plasma activation modification; After the plasma activation modification, the piezoelectric substrate is subjected to nanoscale chemical mechanical polishing; After the nanoscale chemical mechanical polishing, the piezoelectric substrate is washed by using deionized water, and the piezoelectric substrate is dried by using nitrogen after the washing.

3. The preparation method according to claim 2, characterized in that, During the plasma activation modification, oxygen is introduced into the vacuum plasma treatment instrument.

4. The method of claim 1, wherein, Before the electrode metal film is deposited by using the magnetron sputtering method, the titanium target and the gold target are subjected to pre-sputtering treatment.

5. The production method according to claim 1, characterized by, During the first glue coating and patterning, the first glue is selected to be polyimide photoresist; the first glue is coated on the surface of the electrode metal film by using a spin coating mode; after the first glue coating, front baking treatment is carried out; after the front baking, the first glue is exposed by using an ultraviolet photoetching machine through a finger electrode mask plate; After the exposure, the first glue is developed by using a developing solution; After the development, the first glue is washed by using deionized water; after the washing, the first glue is subjected to hard baking treatment.

6. The method of claim 1, wherein, The interface modification layer is prepared by using a vapor deposition method; the material of the interface modification layer is selected to be silane coupling agent; during the vapor deposition, the vacuum degree is not higher than 1*10-3 Pa; the thickness of the interface modification layer is controlled to be between 10 nm and 20 nm.

7. The preparation method according to claim 1, characterized in that, During the second glue coating and first glue removal, the second glue is selected to be nanoimprint photoresist; the second glue is coated by using a spin coating mode; after the second glue coating, post-baking treatment is carried out; when the first glue is removed, the substrate is soaked in N-methyl pyrrolidone solution; During the first glue removal, ultrasonic assistance is adopted.

8. The method of claim 1, wherein, During the etching, the concentration closed-loop control is achieved by using an ultraviolet-visible spectrophotometer to measure the absorbance of the etching liquid in real time; According to the Lambert-Beer law, the concentration of the etching liquid is calculated, and the expression of the Lambert-Beer law is A(lambda)=epsilon(lambda)*C(t)*L, wherein A(lambda) is the absorbance of the etching liquid, epsilon(lambda) is the molar absorption coefficient of the etching liquid, C(t) is the molar concentration of the etching liquid at t moment, and L is the optical path length; when the calculated C(t) is less than 90% of the initial concentration, the etching liquid mother liquor is supplemented by using a second micro-feeding device in a program-controlled double micro-feeding device; the initial concentration is calibrated by using an inductively coupled plasma emission spectrometer.

9. The method of claim 1, wherein, The variable speed rotating control of the film-distributing machine adopts a three-stage speed curve; the expression of the three-stage speed curve is: when 0≤t≤t1, n(t)=n1+(n2-n1)×(t / t1); when t1<t≤t2, n(t)=n2; when t2<t≤t 总 , n(t)=n2-(n2-n1)×((t-t2) / (t 总 -t2)), wherein n(t) is the rotating speed of the film-distributing machine at t moment, n1 is the initial rotating speed of the film-distributing machine, n2 is the maximum rotating speed of the film-distributing machine, t 总 is the total corrosion time, t1 is the acceleration time, and t2 is the uniform speed end time.

10. A high-stability surface acoustic wave strain sensor, characterized by, The surface acoustic wave strain sensor comprises a weighted interdigital electrode, a piezoelectric substrate and a package tube.

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