Flexible electrode structure, manufacturing method thereof and miniature soil humidity sensor

Abstract

The invention relates to the technical field of electrodes and sensors, and discloses a flexible electrode structure which comprises a first outer protection layer, a first electrode layer, an organic polymer layer, a second electrode layer and a second outer protection layer which are sequentially arranged from bottom to top. The organic polymer layer is made of a PI material; the first outer protection layer and the second outer protection layer are ion exchange membranes made of chitosan and perfluorinated sulfonic acid resin materials. In addition, the invention further discloses a manufacturing method of the flexible electrode structure and a miniature soil humidity sensor, and a measuring probe of the sensor adopts the flexible electrode structure. The soil humidity sensor is more miniaturized and can be more conveniently mounted in soil, damage to the soil structure is reduced, and the soil humidity sensor is more convenient to carry and use.

Description

Technical Field

[0001] The present invention relates to the technical field of electrodes and sensors, and in particular to a flexible electrode structure, a preparation method thereof and a micro soil moisture sensor. Background Art

[0002] In agricultural production, accurate soil moisture information is essential for reasonable irrigation. Traditional irrigation methods often rely on experience, which can easily lead to insufficient or excessive irrigation. Insufficient irrigation will affect the growth and yield of crops, while excessive irrigation will waste water resources and may also lead to problems such as soil salinization. Different crops have different requirements for soil moisture at different growth stages. For example, during the seed germination period, higher soil moisture is required to ensure seed germination; in the later growth period of crops, overly wet soil may cause pests and diseases. Real-time monitoring of soil moisture by soil moisture sensors can provide agricultural producers with accurate soil moisture information so that they can carry out scientific planting management according to crop needs, optimize crop growth environment, and improve crop quality and yield. Therefore, in modern agricultural production, a device that can accurately measure soil moisture is needed to provide a basis for scientific irrigation. Soil moisture sensors came into being to help farmers carry out precise irrigation according to the actual soil moisture conditions, improve water resource utilization efficiency, and ensure the growth quality and yield of crops.

[0003] Traditional rigid sensors may cause some damage to the soil structure when inserted into the soil, such as squeezing soil particles and forming pores, which may affect the air permeability and water permeability of the soil and the growth of plant roots; and in agricultural production and field environment monitoring, the sensor may be affected by various mechanical external forces, such as collisions with farm tools and activities of soil animals. In recent years, with the continuous development of micro-electromechanical systems (MEMS) technology, soil moisture sensors have the possibility of developing in the direction of miniaturization and flexibility, which can be used to solve the above problems. Summary of the invention

[0004] In view of this, the present invention provides a flexible electrode structure, a manufacturing method thereof and a micro soil moisture sensor, so that the soil moisture sensor is more miniaturized and more convenient to install in the soil, reducing damage to the soil structure and being easier to carry and use.

[0005] The present invention is achieved through the following technical solutions: a flexible electrode structure, comprising a first outer protective layer, a first electrode layer, an organic polymer layer, a second electrode layer, and a second outer protective layer arranged in sequence from bottom to top.

[0006] Furthermore, the organic polymer layer is made of PI material; the first outer protective layer and the second outer protective layer are both ion exchange membranes made of chitosan and perfluorosulfonic acid resin materials.

[0007] Furthermore, the first electrode layer is made of copper-chromium alloy, and the second electrode layer is made of copper.

[0008] A method for manufacturing a flexible electrode structure comprises the following steps: step 1: using a silicon wafer as a carrier; step 2: sequentially spin coating a first layer of PMMA, a first layer of PI and a first layer of photoresist on the surface of the silicon wafer; step 3: using photolithography technology, exposing and imprinting the electrode shape onto the first layer of photoresist through a mask, and magnetron sputtering a first layer of metal on the surface of the first layer of photoresist; step 4: spin coating a second layer of PI on the surface of the first layer of metal, magnetron sputtering a second layer of metal, and then spin coating a second layer of photoresist; step 5: using photolithography technology again, exposing and imprinting the electrode shape onto the second layer of photoresist through a mask, and then using a developer to remove the unexposed parts of the first layer of photoresist and the second layer of photoresist, and the remaining parts of the first layer of photoresist and the second layer of photoresist The remaining parts are all in the shape of electrodes; Step six: Use etching solution to remove the parts of the first layer of metal and the second layer of metal that are not protected by the remaining parts of the first layer of photoresist and the second layer of photoresist, and the remaining parts of the first layer of metal and the second layer of metal are all in the shape of electrodes; Step seven: Use acetone solution to remove the remaining parts of the first layer of photoresist and the second layer of photoresist; Step eight: Use a reactive ion etching system to remove the first layer of PI and the parts of the second layer of PI that are not protected by the remaining parts of the first layer of metal and the second layer of metal, and the remaining parts of the second layer of PI are in the shape of electrodes, thereby obtaining a PI-metal flexible structure, which can now be separated from the silicon wafer; Step nine: Use an ion exchange membrane to encapsulate the PI-metal flexible structure to obtain a flexible electrode structure.

[0009] Furthermore, in step 2, the first layer of PI uses PI with a viscosity of 15%; in step 3, the first layer of metal uses copper-chromium alloy, and when magnetron sputtering the first layer of metal, a first layer of chromium with a thickness of 50nm and a first layer of copper with a thickness of 200nm are magnetron sputtered in sequence; in step 4, the second layer of PI uses PI with a viscosity of 10%, the second layer of metal uses copper, and when magnetron sputtering the second layer of metal, a second layer of copper with a thickness of 200nm is magnetron sputtered.

[0010] Furthermore, the ion exchange membrane is made of chitosan and perfluorosulfonic acid resin materials.

[0011] A miniature soil moisture sensor comprises a measuring probe, a measuring conversion circuit and a data transmission module; the measuring probe adopts the flexible electrode structure described above and is used to obtain the dielectric constant of the soil; the measuring conversion circuit is used to convert the dielectric constant of the soil into soil moisture; and the data transmission module is used to transmit the soil moisture to a terminal.

[0012] Compared with the prior art, the present invention has the following beneficial effects:

[0013] 1. The flexible electrode structure of the present invention is small in size and light in weight. It is very convenient to bury in the soil or fix on the soil surface. It does not require complicated installation tools and equipment. The soil structure is minimally affected and the natural state of the soil can be maintained. The use of organic materials improves the corrosion resistance of the flexible electrode structure of the sensor and ensures its service life in the field. The micro soil moisture sensor is small in size and can operate with low power consumption. Low power consumption extends the battery life, reduces the frequency of battery replacement and maintenance costs, and can work continuously and stably in some remote areas or places where it is difficult to access an external power supply, ensuring the continuity of soil moisture monitoring.

[0014] 2. The flexible electrode structure of the present invention has certain elasticity and toughness, can withstand a certain degree of mechanical impact and extrusion, and is not easily damaged; and has a relatively large area and is more sensitive to changes in capacitance, which enables the sensor to respond quickly to changes in soil moisture and has a shorter response time.

[0015] 3. The present invention uses an ion exchange membrane made of chitosan and perfluorosulfonic acid resin materials as a protective layer package, which can not only ensure that ions in the soil pass through the outer protective layer, but also further ensure the elasticity and toughness of the flexible electrode structure.

[0016] 4. The micro soil moisture sensor of the present invention has high low power consumption characteristics, consumes very little power during operation, and can work for a long time relying on battery power. Compared with traditional high-power consumption sensors, it reduces the dependence on external power supply, reduces the cost of wiring and power supply equipment, and also avoids the problem of monitoring interruption caused by power supply line failure. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 Schematic diagram of electrode shape.

[0018] Figure 2 The manufacturing process of flexible electrode structure. DETAILED DESCRIPTION

[0019] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0020] The present invention provides a flexible electrode structure, comprising a first outer protective layer, a first electrode layer, an organic polymer layer, a second electrode layer, and a second outer protective layer arranged in sequence from bottom to top. The organic polymer layer is made of PI (Polyimide) material; the first outer protective layer and the second outer protective layer are both ion exchange membranes made of chitosan and perfluorosulfonic acid resin materials; the first electrode layer is made of copper-chromium alloy, and the second electrode layer is made of copper.

[0021] The present invention provides a method for manufacturing a flexible electrode structure, such as Figure 2 As shown, the following steps are included:

[0022] Step 1: Use a silicon wafer as a carrier. In this embodiment, a silicon wafer with a thickness of 400 μm is used.

[0023] Step 2: Spin-coat the first layer of PMMA (acrylic), the first layer of PI and the first layer of photoresist on the surface of the silicon wafer in sequence, PMMA is used to isolate the silicon wafer and the first layer of PI; specifically, the first layer of PI uses polyimide with a viscosity of 15%, and the first layer of photoresist uses AZ5214E ​​positive photoresist. After the AZ5214E ​​positive photoresist is exposed, its unexposed part melts in the developer, and its exposed part is insoluble in the developer but soluble in acetone solution.

[0024] Step 3: Use photolithography technology (first photolithography) to expose and engrave the electrode shape onto the first layer of photoresist through a mask, and magnetron sputter a first layer of chromium with a thickness of 50nm and a first layer of copper with a thickness of 200nm on the surface of the first layer of photoresist in sequence. The first layer of chromium and the first layer of copper constitute a copper-chromium alloy layer.

[0025] Step 4: Spin-coat a second layer of PI (polyimide viscosity 10%) on the surface of the first layer of copper, then magnetron sputter a second layer of copper with a thickness of 200nm, and then spin-coat a second layer of photoresist; the second layer of photoresist is made of the same material type as the first layer of photoresist, and uses AZ5214E ​​positive photoresist. In the specific implementation, in this step, an acetone solution can also be used to remove excess photoresist that overflows the silicon wafer.

[0026] Commonly used materials for measuring probes of existing soil moisture sensors include stainless steel, copper alloys and graphite. The common characteristics of the above materials are that they are both conductive and corrosion resistant. Metal conductors are easily corroded, among which copper alloys have better corrosion resistance than stainless steel, while graphite has good corrosion resistance but poor strength and poor conductivity. After experimental comparison, the present invention uses copper and copper-chromium-gold as electrode materials for the measuring probe.

[0027] Step 5: Use photolithography technology again (second photolithography) to expose and imprint the electrode shape onto the second layer of photoresist through a mask, use a developer to remove the unexposed portions of the first layer of photoresist and the second layer of photoresist, and leave the exposed portions of the first layer of photoresist and the second layer of photoresist, that is, the remaining portions of the first layer of photoresist and the second layer of photoresist are all electrode shapes. The electrode shapes in this embodiment are as shown in FIG. Figure 1 As shown, in the specific implementation, the corresponding mask can be made according to the actual electrode shape required; the developer can be ZX238. At this time, the silicon wafer is in turn: the first layer of PMMA, the first layer of PI, the first layer of photoresist exposed part (electrode shape), the copper-chromium alloy layer, the second layer of PI, the second layer of copper, and the second layer of photoresist exposed part (electrode shape).

[0028] Step 6: Use copper etching solution to remove the copper-chromium alloy layer and the second copper layer that are not protected by the first photoresist layer and the remaining part of the second photoresist layer (the exposed part, which is also the electrode shape), that is, the copper-chromium alloy layer and the second copper layer that are protected by the first photoresist layer and the remaining part of the second photoresist layer (the exposed part, which is also the electrode shape) are left, and the copper-chromium alloy layer and the remaining part of the second copper layer are both in the shape of electrodes. At this time, the silicon wafer is in the following order: the first PMMA layer, the first PI layer, the exposed part of the first photoresist layer (electrode shape), the remaining part of the copper-chromium alloy layer (electrode shape), the second PI layer, the remaining part of the second copper layer (electrode shape), and the exposed part of the second photoresist layer (electrode shape).

[0029] Step 7: Use acetone solution to remove the first layer of photoresist and the remaining part of the second layer of photoresist; at this time, the silicon wafer is sequentially: the first layer of PMMA, the first layer of PI, the remaining part of the copper-chromium alloy layer (electrode shape), the second layer of PI, and the remaining part of the second layer of copper (electrode shape).

[0030] Step 8: Use a reactive ion etching system (RIE) to remove the first layer of PI and the portion of the second layer of PI that is not protected by the copper-chromium alloy layer and the remaining portion of the second layer of copper (electrode shape), that is, the portion of the second layer of PI that is protected by the copper-chromium alloy layer and the remaining portion of the second layer of copper (electrode shape) is left, and the remaining portion of the second layer of PI is the electrode shape, and finally a PI-Cu flexible structure is obtained. At this time, the PI-Cu flexible structure can be separated from the silicon wafer, that is, after removing the first layer of PI, the remaining portion of the copper-chromium alloy layer can be peeled off from the silicon wafer with the first layer of PMMA. The PI-Cu flexible structure is: the remaining portion of the copper-chromium alloy layer (electrode shape), the second layer of PI (electrode shape), and the remaining portion of the second layer of copper (electrode shape).

[0031] In a specific implementation, when the copper-chromium alloy layer and the second copper layer are etched with a copper etching solution in step six, it is possible that the copper-chromium alloy layer and the second copper layer are not completely removed, that is, the remaining parts of the copper-chromium alloy layer and the second copper layer have excess copper or copper-chromium alloy in addition to the electrode shape part, but the excess copper or copper-chromium alloy part is particularly thin and small. In step eight, after the reactive ion etching system (RIE) is used to remove the PI, the copper etching solution can be used again to remove the excess copper or copper-chromium alloy.

[0032] Step nine: Use an ion exchange membrane to encapsulate the PI-Cu flexible structure to obtain a flexible electrode structure. Specifically, the ion exchange membrane is made of chitosan and perfluorosulfonic acid resin materials.

[0033] The present invention provides a miniature soil moisture sensor, comprising a measuring probe, a measuring conversion circuit and a data transmission module; the measuring probe adopts the flexible electrode structure described above, and is used to obtain the dielectric constant of the soil based on the FDR electromagnetic wave detection method; the measuring conversion circuit is used to convert the dielectric constant of the soil into soil moisture; and the data transmission module is used to transmit the soil moisture to a terminal.

[0034] The principle of soil moisture sensor is mainly based on the frequency domain reflectometry (FDR) principle, which uses electromagnetic waves to measure the dielectric constant of soil, thereby inferring the water content of soil; the traditional sensor uses metal needles as measuring probes, which are destructive to the soil and are more likely to break and break when subjected to external forces. The present invention uses a flexible electrode structure as a measuring probe, which has little impact on the soil structure during installation and use, can maintain the natural state of the soil, and is conducive to the normal growth of plants; the flexible electrode structure has certain elasticity and toughness, can withstand a certain degree of mechanical shock and extrusion, and is not easy to be damaged; and the relative area of ​​the flexible electrode structure is much larger than that of the metal needle, which is more sensitive to changes in capacitance, which enables the micro soil moisture sensor to respond quickly to changes in soil moisture and has a shorter response time. Whether it is the rapid change of soil moisture during irrigation or the subtle fluctuation of soil moisture in the natural environment, the micro soil moisture sensor with a flexible electrode structure can capture it in time and provide users with real-time humidity information.

[0035] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A flexible electrode structure, characterized in that: The invention comprises a first outer protective layer, a first electrode layer, an organic polymer layer, a second electrode layer and a second outer protective layer which are arranged in sequence from bottom to top.

2. The flexible electrode structure according to claim 1, characterized in that: The organic polymer layer adopts PI material; the first outer protective layer and the second outer protective layer are both ion exchange membranes made of chitosan and perfluorosulfonic acid resin materials.

3. The flexible electrode structure according to claim 1 or 2, characterized in that: The first electrode layer is made of copper-chromium alloy, and the second electrode layer is made of copper.

4. A method for manufacturing a flexible electrode structure, characterized in that: The following steps are involved: Step 1: Using silicon wafer as carrier; Step 2: Spin coating the first layer of PMMA, the first layer of PI and the first layer of photoresist on the surface of the silicon wafer in sequence; Step 3: Use photolithography technology to expose and print the electrode shape onto the first layer of photoresist through a mask, and magnetron sputter the first layer of metal on the surface of the first layer of photoresist; Step 4: Spin-coat a second layer of PI on the surface of the first metal layer, then magnetron sputter the second metal layer, and then spin-coat a second layer of photoresist; Step 5: Use photolithography technology again to expose and print the electrode shape onto the second layer of photoresist through a mask, and then use a developer to remove the unexposed parts of the first layer of photoresist and the second layer of photoresist, and the remaining parts of the first layer of photoresist and the second layer of photoresist are all electrode shapes; Step 6: Use an etching solution to remove the portions of the first metal layer and the second metal layer that are not protected by the remaining portions of the first photoresist layer and the second photoresist layer, and the remaining portions of the first metal layer and the second metal layer are all in the shape of electrodes; Step 7: Use acetone solution to remove the first layer of photoresist and the remaining part of the second layer of photoresist; Step 8: Use a reactive ion etching system to remove the first layer of PI and the portion of the second layer of PI that is not protected by the first layer of metal and the remaining portion of the second layer of metal. The remaining portion of the second layer of PI is in the shape of an electrode, and a PI-metal flexible structure is obtained. At this time, the PI-metal flexible structure can be separated from the silicon wafer; Step nine: Use an ion exchange membrane to encapsulate the PI-metal flexible structure to obtain a flexible electrode structure.

5. The method for manufacturing a flexible electrode structure according to claim 4, characterized in that: In step 2, the first layer of PI uses PI with a viscosity of 15%; In step 3, the first metal layer is made of copper-chromium alloy. When magnetron sputtering the first metal layer, a first chromium layer with a thickness of 50 nm and a first copper layer with a thickness of 200 nm are sequentially magnetron sputtered. In step 4, the second PI layer uses PI with a viscosity of 10%, the second metal layer uses copper, and when magnetron sputtering the second metal layer, the magnetron sputtering second copper layer has a thickness of 200 nm.

6. The method for manufacturing a flexible electrode structure according to claim 4 or 5, characterized in that: The ion exchange membrane is made of chitosan and perfluorosulfonic acid resin materials.

7. A micro soil moisture sensor, characterized in that: It comprises a measuring probe, a measuring conversion circuit and a data transmission module; the measuring probe adopts the flexible electrode structure as described in any one of claims 1 to 6, and is used to obtain the dielectric constant of the soil; the measuring conversion circuit is used to convert the dielectric constant of the soil into soil moisture; the data transmission module is used to transmit the soil moisture to the terminal.

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