Underwater acoustic sensor based on nanocracks and preparation method thereof

Abstract

The invention provides an underwater acoustic sensor based on nanocracks and a preparation method thereof, and belongs to the technical field of hydrophones. The method comprises the following steps: firstly, depositing a layer of metal film on the surface of a flexible polymer, spin-coating a layer of photoresist on the surface of the metal film on the flexible polymer, and patterning the photoresist and the metal film; secondly, patterning the photoresist on the metal film again; thirdly, bending the flexible polymer and the metal film to generate nano cracks on the metal film; and finally, adhering the flexible polymer, polymethyl methacrylate and the glass sheet, injecting pure water into the cavity, and sealing to obtain the underwater acoustic sensor. The underwater acoustic sensor based on the nanocrack manufactured by the invention has ultrahigh sensitivity, and the nanocrack sensing unit with ultrahigh sensitivity and flexibility is adhered to the substrate with the cavity, so that the ultra-sensitive detection of low-frequency underwater sound can be realized, and the detection range of the underwater acoustic sensor is expanded.

Description

Technical Field

[0001] The invention belongs to the technical field of hydrophones and relates to a hydroacoustic sensor based on nanocracks and a preparation method thereof. Background Art

[0002] Low-frequency hydroacoustic signals are commonly used for long-range and deep-sea detection because they are less absorbed in water and have a long propagation distance. Some underwater weapons and submarines use acoustic stealth technology, and the noise level is getting lower and lower, and the noise frequency is also reduced. Therefore, the acquisition and processing of low-frequency hydroacoustic signals in the underwater sound field is a problem that scholars in the fields of ocean acoustics and hydroacoustics are currently paying more attention to. Hydroacoustic target detection technology is also an important research direction in the field of hydroacoustic signal processing and sonar, and is one of the core technologies in marine application fields such as environmental perception, target monitoring, resource exploration, and intelligence collection. Researchers can detect long-range targets and stealth targets by analyzing and processing vector information in the underwater sound field, such as particle velocity, acceleration, displacement, and sound pressure information.

[0003] A hydroacoustic sensor is a device that converts electrical signals into hydroacoustic signals or vice versa. Its role in sonar is similar to that of an antenna in a radio device. It is an acoustic device that transmits and receives sound waves underwater. A transducer that converts acoustic signals into electrical signals is used to receive acoustic signals in water. It is called a receiving transducer, also often called a hydrophone. A hydrophone can convert pressure changes generated underwater into electrical signals. Through further analysis and processing of the 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] At present, the common hydroacoustic sensor sensing units are all made on hard substrates, with low sensitivity and complex structure and manufacturing process. They are usually used to detect relatively high-frequency hydroacoustic signals, and have weak detection capabilities for low-frequency hydroacoustic signals. There is no specific application for using flexible substrates as hydroacoustic sensor sensing units to detect low-frequency hydroacoustic signals. So far, no nanocrack hydroacoustic sensors based on flexible substrates have been reported. Summary of the invention

[0005] In response to the existing problems, the present invention provides an underwater acoustic sensor based on nanocracks and a preparation method thereof. The underwater acoustic sensor has high sensitivity to low-frequency underwater alternating sound pressure. At the same time, the method uses a metal film on a flexible substrate as a sensing sensitive unit, does not require processes such as high-temperature annealing, has a simple manufacturing process and structure, and has high sensitivity to low-frequency underwater sound.

[0006] In order to achieve the above object, the technical solution adopted by the present invention is:

[0007] A method for preparing an underwater acoustic sensor based on nanocracks, wherein the first step of the preparation method is to deposit a metal film on the surface of a flexible polymer by a thin film deposition method; the second step is to spin-coat a layer of photoresist on the surface of the metal film on the flexible polymer, and pattern the photoresist and the metal film; the third step is to pattern the photoresist on the metal film again; the fourth step is to bend the flexible polymer and the metal film to generate nanocracks on the metal film, and remove the photoresist attached to the metal film; the fifth step is to bond the flexible polymer to polymethyl methacrylate (PMMA) cut by laser, and then bond it to a glass sheet, and finally inject pure water into the cavity of the underwater acoustic sensor and seal it to complete the production of the underwater acoustic sensor. Specifically, the following steps are included:

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

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

[0010] Step 3) According to the required nanocrack density on the metal film, the photoresist on the metal film is patterned again to obtain a photoresist pattern with photoresist stripes having the same spacing and arranged horizontally, so that the neutral layers of the flexible polymer with photoresist and the flexible polymer without photoresist are at different levels.

[0011] Step 4) bend the flexible polymer in a direction parallel to the photoresist stripes, so that nanocracks are generated in the metal film on the flexible polymer. In this step, since part of the metal film has photoresist and part of the metal film does not have photoresist, after bending, nanocracks are generated in the part of the metal film not covered by the photoresist, that is, due to the protective effect of the photoresist stripes, nanocracks are only generated in the metal film area not covered by the photoresist, so as to accurately control the position and crack density of the nanocracks.

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

[0013] Step 6) A hole penetrating the PMMA substrate of the same size as the flexible polymer is cut through the middle of the PMMA substrate by a laser cutting machine, and the shape of the hole is not restricted. The flexible polymer is bonded to one side of the PMMA substrate after the cut by using shadowless adhesive, and some areas are reserved for non-bonding to facilitate subsequent operations. Shadowless adhesive is applied to the other surface of the PMMA substrate after the cut, and a glass sheet is bonded to the other side of the PMMA substrate. At this time, a cavity is formed in the space after the PMMA substrate is cut between the flexible polymer and the glass sheet. Liquid is injected into the cavity through the aforementioned non-bonded area using a syringe to balance part of the pressure, and the edges of the non-bonded area are bonded again by using shadowless adhesive to complete the production of the underwater acoustic sensor.

[0014] Furthermore, the patterning method in step 2) is a standard photolithography process and wet etching. The specific method is: using an ultraviolet photolithography machine to pattern the photoresist on the flexible polymer, using 0.5% sodium hydroxide solution as a developer to remove the exposed photoresist, the unexposed photoresist is used as a protective layer, and using a gold etching solution 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, the standard photolithography process is used to expose after replacing the mask, and 0.5% sodium hydroxide solution is used as a developer to remove the exposed photoresist, and the photoresist pattern is completed, and the obtained photoresist pattern is a horizontally arranged photoresist stripe with the same spacing.

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

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

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

[0019] A nanocrack-based hydroacoustic sensor is prepared by the above-mentioned preparation method. In the hydroacoustic sensor based on nanocracks prepared by the present invention, the resistance of the metal film with nanocracks on the surface of the flexible polymer changes with the change of low-frequency underwater alternating pressure, and the detection of low-frequency hydroacoustic can be achieved by connecting a resistance measuring instrument to the electrode using a wire.

[0020] Compared with the existing hydrophone manufacturing method, the present invention has the following beneficial effects:

[0021] (1) The present invention produces a metal film with nanocracks on a flexible polymer. In the absence of external strain, the edges of the nanocracks can connect with the adjacent crack edges to transmit electrons. Under uniaxial tension perpendicular to the crack direction, the gap distance between each pair of crack edges increases until it exceeds the rough height of the crack and disconnects. At the same time, due to the Poisson effect, these edges will shrink in the direction perpendicular to the tensile force and reconnect with their adjacent edges. As the strain increases, the number of connected edges gradually decreases until all edges are disconnected. Therefore, this "disconnection-reconnection" process produces a change in the resistance of the nanocrack hydroacoustic sensor. Through the continuous change of underwater sound pressure, the nanocracks on the surface of the flexible polymer will undergo this "disconnection-reconnection" process, thereby achieving ultra-sensitive detection of low-frequency hydroacoustics and improving the detection range of the hydroacoustic sensor.

[0022] (2) The present invention fills the gap in the field of hydrophone technology in that there is no hydrophone sensor made of flexible polymer.

[0023] (3) In addition, the method used in the present invention is time-saving, has a concise and efficient process flow and a simple structure, can significantly improve the low-frequency underwater acoustic resolution, and can effectively promote the development of my country's new marine security technology and the improvement of detection capabilities. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 It is a metal film deposited on the surface of a flexible polymer;

[0025] Figure 2 The photoresist is spin-coated on the surface of the metal film and the photoresist and the metal film are patterned;

[0026] Figure 3 The photoresist on the surface of the metal film is patterned again;

[0027] Figure 4 It is the bending of flexible polymers that produces nanocracks;

[0028] Figure 5 It is to remove the photoresist on the surface of the metal film;

[0029] Figure 6 It is the adhesion of flexible polymer to polymethyl methacrylate (PMMA) substrate;

[0030] Figure 7 The above-mentioned 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] Fig. 9 It is the resistance change diagram of the hydroacoustic sensor at a frequency of 5-20Hz; Fig. 9 (a) is the resistance change diagram of the underwater acoustic sensor at a frequency of 5 Hz; Fig. 9 (b) is the resistance change diagram of the underwater acoustic sensor at a frequency of 10 Hz; Fig. 9 (c) is the resistance change diagram of the underwater acoustic sensor at a frequency of 15 Hz; Fig. 9 (d) is the resistance change diagram of the underwater acoustic sensor at a frequency of 20 Hz.

[0033] In the figure: 1 flexible polymer; 2 metal film; 3 photoresist; 4 polyimide film; 5 metal nanocracks; 6 polymethyl methacrylate (PMMA); 7 glass sheet. DETAILED DESCRIPTION

[0034] The embodiments of the present invention are described in detail below in combination with the technical solutions and the accompanying drawings.

[0035] As attached Figure 1-Figure 7 As shown, the method for preparing the nanocrack-based hydroacoustic sensor provided in this embodiment is as follows:

[0036] Step 1) As attached Figure 1 As shown, a 50 nm thick metal film 2 (Au) is sputtered on 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) is spin-coated on the surface of the metal film 2, and the spin-coating parameters are 1000rpm and 30s; the spin-coated flexible polymer is placed at room temperature (25°C) for four hours instead of the pre-baking process; the photoresist 3 is exposed to patterning using an ultraviolet lithography machine, and the exposure time is 222.2s, and the light intensity is 2.7mW / cm 2 , then developed in a 0.5% sodium hydroxide solution for 30 seconds, and then placed the flexible polymer at room temperature (25°C) for four hours instead of a post-baking process; 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, and then washed with deionized water.

[0038] Step 3) As attached Figure 3 As shown, after replacing the mask, the UV lithography machine is used again to expose the photoresist 3 to achieve patterning. The exposure time is 222.2s and the light intensity is 2.7mW / cm 2 Then, the photoresist 3 is patterned after being developed in a 0.5% by mass 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: the patterned flexible polymer is fixed on a polyimide film 4 (PI) with a thickness of 75 microns, and a stepper motor is used to drive the polyimide film 4 (PI) to rotate 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 not covered by the photoresist.

[0040] Step 5) As attached Figure 5 As shown, the photoresist 3 is exposed by an ultraviolet photolithography machine, and the exposed photoresist 3 is removed by a sodium hydroxide solution having a mass fraction of 0.5%.

[0041] Step 6) As attached Figure 6 , Attachment Figure 7 As shown, a hole penetrating the substrate is cut in the middle of a polymethyl methacrylate (PMMA) substrate of the same size as the flexible polymer by a laser cutting machine, and the shape of the hole is not restricted. The flexible polymer is bonded to one side of the cut polymethyl methacrylate (PMMA) substrate by shadowless adhesive, and some areas are reserved for non-bonding to facilitate subsequent operations. Shadowless adhesive is applied to the other surface of the cut polymethyl methacrylate (PMMA) substrate, and a glass sheet is bonded to the other side of the polymethyl methacrylate (PMMA) substrate. At this time, a cavity is formed in the space after the polymethyl methacrylate (PMMA) substrate is cut between the flexible polymer and the glass sheet. Liquid is injected into the cavity through the aforementioned non-bonded area using a syringe to balance part of the pressure, and the edges of the non-bonded area are bonded again by shadowless adhesive to complete the production of the underwater acoustic sensor.

[0042] The above-described embodiments merely express the implementation methods of the present invention, but they cannot be understood as limiting the scope of the patent of the present invention. It should be pointed out that for those skilled in the art, several variations and improvements can be made to the specification without departing from the concept of the present invention, which all belong to the protection scope of the present invention.

Claims

1. A method for preparing a hydroacoustic sensor based on nanocracks, characterized in that: The preparation method comprises the following steps: Step 1) sputtering a metal film on the surface of the flexible polymer using a thin film deposition method; Step 2) spin coating a layer of photoresist on the surface of the metal film, and patterning the photoresist and the metal film; Step 3) according to the required nanocrack density on the metal film, the photoresist on the metal film is patterned again, and the obtained photoresist pattern is photoresist stripes with the same spacing and arranged horizontally, so that the neutral layer of the part of the flexible polymer with photoresist and the part of the flexible polymer without photoresist are at different levels; Step 4) bending the flexible polymer in a direction parallel to the photoresist stripes, so that nanocracks are generated in the metal film on the flexible polymer; Step 5) removing the photoresist on the metal film to obtain a strip-shaped sensing unit with nanocracks, wherein both ends of the strip-shaped sensing unit are electrodes of the sensing unit, and there are no cracks at the electrodes; Step 6) A through hole penetrating the middle of a polymethyl methacrylate (PMMA) substrate of the same size as the flexible polymer is cut by a laser cutting machine; the flexible polymer is bonded to one side of the cut polymethyl methacrylate (PMMA) substrate by using a shadowless adhesive, and a partial area is reserved for non-bonding; a glass sheet is bonded to the other side of the polymethyl methacrylate (PMMA) substrate; a cavity is formed at the through hole in the middle of the polymethyl methacrylate (PMMA) substrate as a cavity for the underwater acoustic sensor, and after liquid is injected into the cavity through the non-bonding area, the edge of the non-bonding area is bonded to complete the production of the underwater acoustic sensor.

2. The method for preparing a hydroacoustic sensor based on nanocracks according to claim 1, characterized in that: In the step 1), the substrate of the flexible polymer is polydimethylsiloxane (PDMS), and the metal film is gold (Au); and the film deposition method is magnetron sputtering.

3. The method for preparing a hydroacoustic sensor based on nanocracks according to claim 1, characterized in that: The patterning method in step 2) is a standard photolithography process and wet etching; the photoresist is BP212 positive photoresist.

4. The method for preparing a hydroacoustic sensor based on nanocracks according to claim 1, characterized in that: The method for re-patterning in step 3) is a standard photolithography process.

5. The method for preparing a hydroacoustic sensor based on nanocracks according to claim 1, characterized in that: In the step 4), the specific method for bending the flexible polymer to generate nanocracks in the metal film is: the flexible polymer is fixed on the polyimide film, and the polyimide film is driven by a motor to be wound around a steel rod with a curvature radius, so that the metal film generates nanocracks parallel to the direction of the steel rod. The curvature radius of the steel rod is selected according to different nanocrack densities.

6. The method for preparing a hydroacoustic sensor based on nanocracks according to claim 1, characterized in that: The developer in step 5) is a 0.5% by mass sodium hydroxide solution.

7. The method for preparing a hydroacoustic sensor based on nanocracks according to claim 1, characterized in that: The liquid in step 6) is pure water.

8. A hydroacoustic sensor based on nanocracks, characterized in that: The hydroacoustic sensor is made by the preparation method described in any one of claims 1 to 7, and can be connected to a resistance measuring instrument to detect low-frequency hydroacoustic sound.

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