A nano-crack-based underwater acoustic sensor and a preparation method thereof

Abstract

The application provides a kind of nanometer crack-based underwater acoustic sensor and its preparation method, belongs to hydrophone technical field.The method first, in the flexible polymer surface deposition a layer of metal film, spin coating a layer of photoresist on the metal film surface of flexible polymer, photoresist and metal film patterning;Second, the photoresist on the metal film is again patterned;Third, the flexible polymer and metal film are bent to produce nanometer crack on the metal film;Finally, the flexible polymer, polymethyl methacrylate, glass sheet are bonded, and after injecting pure water in the cavity, it is sealed to obtain underwater acoustic sensor.The nanometer crack-based underwater acoustic sensor manufactured by the application has ultra-high sensitivity, by bonding the nanometer crack sensing unit with ultra-high sensitivity and flexibility on the substrate with cavity, the ultra-sensitive detection of low-frequency underwater acoustic can be realized, and the detection range of the underwater acoustic sensor is improved.

Description

Technical Field

[0001] This invention belongs to the field of hydrophone technology and relates to a hydroacoustic sensor based on nanocracks and its preparation method. Background Technology

[0002] Low-frequency underwater acoustic signals, due to their low absorption in water and long propagation distance, have become a common means of long-range and deep-sea detection. Some underwater weapons and submarines employ acoustic stealth technology, resulting in increasingly lower noise levels and frequencies. Therefore, the acquisition and processing of low-frequency underwater acoustic signals in underwater sound fields is a major focus for researchers in the fields of marine acoustics and underwater acoustics. Underwater acoustic target detection technology is also an important research direction in the fields of underwater acoustic signal processing and sonar, and is one of the core technologies in marine applications such as environmental perception, target monitoring, resource exploration, and intelligence gathering. Researchers can detect long-range and stealth targets by analyzing and processing vector information in underwater sound fields, such as particle velocity, acceleration, displacement, and sound pressure information.

[0003] An underwater acoustic sensor is a device that converts electrical signals into underwater acoustic signals or vice versa. Its role in sonar is similar to that of an antenna in radio equipment; it is an acoustic device that transmits and receives sound waves underwater. The transducer that converts acoustic signals into electrical signals, used to receive acoustic signals in the water, is called a receiving transducer, also commonly known as a hydrophone. Hydrophones can convert underwater pressure changes into electrical signals. Through further analysis and processing of these electrical signals, the true changes in the underwater low-frequency sound field can be obtained. Low-frequency hydrophones play an extremely important role in national defense and security.

[0004] Currently, most common underwater acoustic sensors are fabricated on rigid substrates, resulting in low sensitivity and complex structures and manufacturing processes. Furthermore, they are typically designed for detecting higher-frequency underwater acoustic signals, with limited ability to detect low-frequency signals. There are currently no concrete applications of using flexible substrates as sensing units for detecting low-frequency underwater acoustic signals. No nanocrack underwater acoustic sensors based on flexible substrates have been reported to date. Summary of the Invention

[0005] To address existing problems, this invention provides a nanocrack-based underwater acoustic sensor and its fabrication method. This underwater acoustic sensor exhibits high sensitivity to low-frequency underwater alternating sound pressure. Furthermore, the method uses a metal thin film on a flexible substrate as the sensing element, eliminating the need for high-temperature annealing and other processes. The fabrication process and structure are simple, and it demonstrates high sensitivity to low-frequency underwater acoustics.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0007] A method for fabricating an underwater acoustic sensor based on nanocracks includes the following steps: First, a metal thin film is deposited on the surface of a flexible polymer using a thin-film deposition method. Second, a layer of photoresist is spin-coated onto the metal thin film on the flexible polymer to pattern the photoresist and the metal thin film. Third, the photoresist on the metal thin film is patterned again. Fourth, the flexible polymer and the metal thin film are bent to generate nanocracks on the metal thin film, and the photoresist adhering to the metal thin film is removed. Fifth, the flexible polymer is bonded to laser-cut polymethyl methacrylate (PMMA), then bonded to a glass sheet, and finally, pure water is injected into the cavity of the underwater acoustic sensor and sealed to complete the fabrication of the underwater acoustic sensor. The specific steps include:

[0008] Step 1) A metal thin film is sputtered onto the surface of a flexible polymer using a thin film deposition method. The substrate of the flexible polymer is polydimethylsiloxane (PDMS), and the metal thin film is gold (Au). The thin film deposition method is magnetron sputtering.

[0009] Step 2) Spin-coat a layer of photoresist onto the surface of the metal film and pattern the photoresist and the metal film.

[0010] Step 3) Based on the required nanocrack density on the metal film, the photoresist on the metal film is repatterned to obtain a photoresist pattern of horizontally arranged stripes with the same spacing, thereby realizing that the part of the flexible polymer with photoresist and the part of the flexible polymer without photoresist are at different levels.

[0011] Step 4) The flexible polymer is bent along a direction parallel to the photoresist stripes, thereby inducing nanocracks in the metal film on the flexible polymer. In this step, since some parts of the metal film have photoresist while others do not, after bending, nanocracks are generated in the parts of the metal film not covered by photoresist. That is, due to the protective effect of the photoresist stripes, nanocracks only occur in the areas of the metal film not covered by photoresist, thus accurately controlling the location and crack density of the nanocracks.

[0012] Step 5) Remove the photoresist from the metal film to obtain a strip-shaped sensing unit with nanocracks, wherein the two ends of the strip-shaped sensing unit are electrodes of the sensing unit, and there are no cracks at the electrodes. In this step, after fully exposing the photoresist to the flexible polymer using standard photolithography, the photoresist on the surface of the metal film is removed using a developing solution to obtain the strip-shaped sensing unit with nanocracks.

[0013] Step 6) Using a laser cutter, cut a hole through the center of a polymethyl methacrylate (PMMA) substrate of the same size as the flexible polymer. The shape of the hole is unrestricted. Adhere the flexible polymer to one side of the cut PMMA substrate with UV adhesive, leaving a portion unadheded for subsequent operations. Apply UV adhesive to the other surface of the cut PMMA substrate and adhere the glass sheet to the other side of the PMMA substrate. At this point, a cavity will be formed in the space between the flexible polymer and the glass sheet where the PMMA substrate was cut. Inject liquid into this cavity through the unadheded area using a syringe to balance some of the pressure. Adhere the edges of the unadheded area again with UV adhesive to complete the fabrication of the underwater acoustic sensor.

[0014] Furthermore, the patterning method in step 2) is a standard photolithography process and wet etching. Specifically, a UV lithography machine is used to pattern the photoresist on the flexible polymer. A 0.5% sodium hydroxide solution is used as the developer to remove the exposed photoresist, and the unexposed photoresist serves as a protective layer. A gold etching solution is then used to remove the unprotected metal film. The photoresist is BP212 positive photoresist.

[0015] Furthermore, the method for re-patterning in step 3) is a standard photolithography process. Specifically, after changing the mask, exposure is performed using a standard photolithography process. A 0.5% sodium hydroxide solution is used as the developer to remove the exposed photoresist, thus completing the patterning of the photoresist. The resulting photoresist pattern consists of horizontally arranged photoresist stripes with the same spacing.

[0016] Furthermore, in step 4), the specific method for the flexible polymer to bend and cause nanocracks in the metal film is as follows: the flexible polymer is fixed on the polyimide film, and the polyimide film is wound around a steel rod with a certain radius of curvature by a motor. The metal film will generate nanocracks parallel to the direction of the steel rod. The radius of curvature of the steel rod is selected according to the different nanocrack densities.

[0017] Furthermore, the developing solution mentioned in step 5) is a 0.5% sodium hydroxide solution by mass.

[0018] Furthermore, the liquid mentioned in step 6) is pure water.

[0019] A nanocrack-based underwater acoustic sensor is fabricated using the method described above. The nanocrack-based underwater acoustic sensor prepared by this invention utilizes a flexible polymer surface with a metal film containing nanocracks. The resistance of this film changes with variations in low-frequency underwater alternating pressure. By connecting a resistance measuring instrument to the electrodes using wires, low-frequency underwater acoustic signals can be detected.

[0020] Compared with existing hydrophone manufacturing methods, the present invention has the following advantages:

[0021] (1) This invention involves fabricating a metal thin film with nanocracks on a flexible polymer. Under no applied strain, the edges of the nanocracks can connect with adjacent crack edges to transfer electrons. Under uniaxial tension perpendicular to the crack direction, the gap between each pair of crack edges increases until it exceeds the crack roughness height and breaks off. Simultaneously, due to the Poisson effect, these edges contract along the direction perpendicular to the tensile force and reconnect with their adjacent edges. As strain increases, the number of connected edges gradually decreases until all edges break off. Therefore, this "disconnect-reconnect" process generates a resistance change in the nanocrack underwater acoustic sensor. Through continuous changes in underwater sound pressure, the nanocracks on the flexible polymer surface undergo this "disconnect-reconnect" process, thereby achieving ultrasensitive detection of low-frequency underwater sounds and improving the detection range of the underwater acoustic sensor.

[0022] (2) This invention fills the gap in the field of hydrophone technology where there are no underwater acoustic sensors made of flexible polymers.

[0023] (3) In addition, the method used in this invention is time-saving, has a simple and efficient process, and a simple structure. It can significantly improve the low-frequency underwater acoustic resolution and can strongly promote the development of new marine security technologies and the improvement of detection capabilities in my country. Attached Figure Description

[0024] Figure 1 It involves depositing a metal thin film on the surface of a flexible polymer;

[0025] Figure 2 It involves spin-coating photoresist onto the surface of a metal thin film and patterning the photoresist with the metal thin film.

[0026] Figure 3 It involves repatching the photoresist on the surface of a thin metal film;

[0027] Figure 4 Nanocracks are generated in flexible polymers that are bent.

[0028] Figure 5 It removes the photoresist from the surface of the metal thin film;

[0029] Figure 6 It involves bonding a flexible polymer to a polymethyl methacrylate (PMMA) substrate;

[0030] Figure 7 The above refers to adhesion to the glass substrate;

[0031] Figure 8 This is a schematic diagram of the three-dimensional structure of a hydroacoustic sensor based on nanocracks;

[0032] Figure 9 This is a graph showing the resistance variation of an underwater acoustic sensor at frequencies of 5-20Hz. Figure 9 (a) is a graph showing the resistance change of the underwater acoustic sensor at a frequency of 5Hz; Figure 9 (b) is a graph showing the resistance change of the underwater acoustic sensor at a frequency of 10Hz; Figure 9 (c) is a graph showing the resistance change of the underwater acoustic sensor at a frequency of 15Hz. Figure 9 (d) shows the resistance change of the underwater acoustic sensor at a frequency of 20Hz.

[0033] In the figure: 1 Flexible polymer; 2 Metal thin film; 3 Photoresist; 4 Polyimide thin film; 5 Metal nanocracks; 6 Polymethyl methacrylate (PMMA); 7 Glass sheet. Detailed Implementation

[0034] The embodiments of the present invention will be described in detail below with reference to the technical solutions and accompanying drawings.

[0035] As attached Figures 1-7 As shown in the figure, the fabrication method of the underwater acoustic sensor based on nanocracks provided in this embodiment is as follows:

[0036] Step 1) as attached Figure 1 As shown, a 50 nm thick metal film 2 (Au) was sputtered onto the surface of a 1 mm thick flexible polymer 1 polydimethylsiloxane (PDMS) by magnetron sputtering.

[0037] Step 2) as attached Figure 2 As shown, a layer of photoresist 3 (BP212) was spin-coated onto the surface of the metal thin film 2 at 1000 rpm for 30 s. The spin-coated flexible polymer was then placed at room temperature (25°C) for four hours to replace the pre-baking process. The photoresist 3 was then exposed to ultraviolet light to achieve patterning, with an exposure time of 222.2 s and a light intensity of 2.7 mW / cm². 2 Then, the flexible polymer was developed in a 0.5% sodium hydroxide solution for 30 seconds, and then placed at room temperature (25°C) for four hours instead of post-baking. The patterned photoresist was used as a wet etching mask, and the metal film 2 was patterned in a gold etching solution with a ratio of I2:KI:H2O = 1g:5g:50mL for 15 seconds. The film was then washed with deionized water.

[0038] Step 3) as attached Figure 3 As shown, after changing the photomask, the photoresist 3 was exposed again using an ultraviolet lithography machine to achieve patterning. The exposure time was 222.2 s, and the light intensity was 2.7 mW / cm². 2 Then, the photoresist 3 was patterned by developing it in a 0.5% sodium hydroxide solution for 30 seconds to obtain photoresist stripes with the same spacing.

[0039] Step 4) as attached Figure 4 As shown, the flexible polymer 1 is bent. The specific method is as follows: the patterned flexible polymer is fixed on a polyimide film 4 (PI) with a thickness of 75 micrometers. A stepper motor is used to drive the polyimide film 4 (PI) to wind around a steel rod with a curvature radius of 2 mm. Nanocracks 5 parallel to the direction of the steel rod will be generated on the metal film 2 that is not covered by photoresist.

[0040] Step 5) as attached Figure 5 As shown, the photoresist 3 is exposed using an ultraviolet lithography machine, and the exposed photoresist 3 is removed by a 0.5% sodium hydroxide solution.

[0041] Step 6) as attached Figure 6 Appendix Figure 7 As shown, a hole is cut through the middle of a polymethyl methacrylate (PMMA) substrate of the same size as the flexible polymer using a laser cutting machine. The shape of the hole is unrestricted. The flexible polymer is bonded to one side of the cut PMMA substrate using UV adhesive, leaving a portion unbonded for subsequent operations. UV adhesive is then applied to the other surface of the cut PMMA substrate, and a glass sheet is bonded to the other side of the PMMA substrate. At this point, a cavity is formed in the space between the flexible polymer and the glass sheet where the PMMA substrate was cut. Liquid is injected into this cavity through the unbonded area using a syringe to balance some of the pressure. The edges of the unbonded area are then bonded together again using UV adhesive, completing the fabrication of the underwater acoustic sensor.

[0042] The embodiments described above are merely illustrative of the implementation methods of the present invention, but should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements to the specification without departing from the concept of the present invention, and these modifications and improvements all fall within the protection scope of the present invention.

Claims

1. A method for fabricating an underwater acoustic sensor based on nanocracks, characterized in that, The preparation method includes the following steps: Step 1) A metal thin film is sputtered onto the surface of the flexible polymer using a thin film deposition method; Step 2) Spin-coat a layer of photoresist onto the surface of the metal film and pattern the photoresist and the metal film; Step 3) Based on the required nanocrack density on the metal film, the photoresist on the metal film is repatterned to obtain a photoresist pattern of horizontally arranged stripes with the same spacing, so that the neutral layer of the flexible polymer with photoresist and the flexible polymer without photoresist are at different levels. Step 4) The flexible polymer is bent along the direction parallel to the photoresist stripes, causing nanocracks to be generated in the metal film on the flexible polymer. The specific method for generating nanocracks in the metal film by bending the flexible polymer is as follows: the flexible polymer is fixed on a polyimide film, and the polyimide film is wound around a steel rod with a radius of curvature by a motor. The metal film generates nanocracks parallel to the direction of the steel rod. The radius of curvature of the steel rod is selected according to the different nanocrack densities. Step 5) Remove the photoresist on the metal film to obtain a strip-shaped sensing unit with nanocracks, wherein the two ends of the strip-shaped sensing unit are electrodes of the sensing unit, and there are no cracks at the electrodes. Step 6) Using a laser cutting machine, cut a through hole in the middle of a polymethyl methacrylate (PMMA) substrate of the same size as the flexible polymer; bond the flexible polymer to one side of the cut PMMA substrate with UV adhesive, leaving a portion unbonded; bond the glass sheet to the other side of the PMMA substrate; a cavity is formed at the through hole in the middle of the PMMA substrate, which serves as the cavity for the underwater acoustic sensor. After injecting liquid into the cavity through the unbonded area, bond the edges of the unbonded area to complete the fabrication of the underwater acoustic sensor.

2. The method for fabricating a nanocrack-based underwater acoustic sensor according to claim 1, characterized in that, In step 1), the substrate of the flexible polymer is polydimethylsiloxane (PDMS), and the metal film is gold (Au); the film deposition method is magnetron sputtering.

3. The method for fabricating a nanocrack-based underwater acoustic sensor according to claim 1, characterized in that, The patterning method in step 2) is standard photolithography and wet etching; the photoresist is BP212 positive photoresist.

4. The method for fabricating a nanocrack-based underwater acoustic sensor according to claim 1, characterized in that, The method for re-patterning in step 3) is the standard photolithography process.

5. The method for fabricating a nanocrack-based underwater acoustic sensor according to claim 1, characterized in that, In step 5), the developing solution is a 0.5% sodium hydroxide solution.

6. The method for fabricating a nanocrack-based underwater acoustic sensor according to claim 1, characterized in that, The liquid mentioned in step 6) is pure water.

7. A hydroacoustic sensor based on nanocracks, characterized in that, The underwater acoustic sensor is prepared by any of the preparation methods described in claims 1-6, and can be connected to a resistance measuring instrument to detect low-frequency underwater sounds.

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