A planar interdigital capacitive humidity sensor with a double-layer composite film structure and a preparation method thereof

Abstract

This invention discloses a planar interdigitated capacitive humidity sensor with a dual-layer composite membrane structure and its fabrication method. The sensor body includes a substrate, an insulating layer, planar interdigitated electrode pairs, an electrode passivation layer, a humidity-sensitive polymer membrane layer, and a protective membrane layer. The protective membrane layer uses dense Parylene material to construct a nanoscale protective layer, allowing gas molecules to pass freely while effectively blocking dust, aerosols, and salt spray pollutants from direct contact with the core humidity-sensitive membrane, greatly enhancing the sensor's environmental tolerance and long-term stability. Parylene material is hydrophobic with low surface energy. In high-humidity environments, this ultra-thin hydrophobic membrane significantly reduces the adhesion probability and size of microdroplets on the humidity-sensitive membrane surface and possesses a certain self-cleaning ability. This effectively suppresses condensation on the humidity sensor surface and maintains surface cleanliness, enabling the sensor to maintain a good capacitance-humidity linear response relationship in high-humidity environments. This improves the sensor's full-range measurement accuracy, especially in the high-humidity range, and its long-term operational stability in high-humidity environments.

Description

Technical Field

[0001] This invention relates to the field of microelectromechanical systems and sensor technology, specifically to a planar interdigitated capacitive humidity sensor with a double-layer composite film structure and its fabrication method. Background Technology

[0002] Capacitive humidity sensors are widely used due to their advantages such as high sensitivity, fast response, low power consumption, and easy integration. Their core principle is to utilize the change in dielectric constant of the sensitive polymer film with the change in ambient humidity, thereby causing a change in the capacitance value of the sensor. Currently, planar interdigitated capacitive humidity sensors on the market usually have a single layer of humidity-sensitive polymer material directly coated on the electrodes.

[0003] However, traditional humidity sensors have the following drawbacks: (1) When the humidity-sensitive membrane is directly exposed to the environment, dust, oil and other pollutants will adhere to or penetrate into the membrane, causing sensor drift, decreased sensitivity or even failure. In an environment with near-saturated humidity >95%RH, water vapor may condense on the surface of the humidity-sensitive membrane to form microdroplets, causing nonlinear jumps in capacitance value and condensation effect, causing the sensor output characteristics to deviate from linearity, and the measurement accuracy and stability to drop sharply. Especially in long-term high humidity or corrosive gas environments, electrodes, such as aluminum electrodes, are easily corroded, affecting the reliability and lifespan of the sensor. (2) In the prior art, there are solutions that use porous breathable membranes for physical isolation, but these membranes often have deficiencies in terms of density, uniformity or compatibility with micro and nano processes, making it difficult to ensure the rapid passage of water vapor molecules while providing effective protection, thereby affecting the response speed and accuracy of the sensor. (3) In the prior art, although there are improvement schemes such as using noble metal electrodes or depositing passivation layers on the electrodes to improve the reliability of the electrodes, the problem of insufficient detection accuracy and long-term stability of the sensor in high humidity environments still exists because the humidity-sensing membrane is in direct contact with the environment. Although adding a filter membrane to the sensor packaging detection port has a certain protective effect, the filter membrane may still be blocked by dust or damaged in the long term. Moreover, it cannot fundamentally improve the problem of accuracy reduction caused by condensation on the surface of the humidity-sensing membrane in high humidity environments. The long-term stability of the sensor is still insufficient. Summary of the Invention The purpose of this invention is to provide a planar interdigitated capacitive humidity sensor with a double-layer composite membrane structure and its fabrication method, to address the problems mentioned in the background art where the humidity-sensitive membrane is directly exposed to the environment, and contaminants such as dust and oil can adhere to or penetrate the membrane, leading to sensor drift, decreased sensitivity, or even failure. In environments with near-saturation humidity (>95% RH), water vapor may condense on the surface of the humidity-sensitive membrane to form microdroplets, causing nonlinear jumps in capacitance values ​​and resulting in a condensation effect. This causes the sensor's output characteristics to deviate from linearity, and the measurement accuracy and stability to drop sharply. Especially in long-term high-humidity or corrosive gas environments, electrodes, such as aluminum electrodes, are easily corroded, affecting the reliability and lifespan of the sensor. Existing technologies employ porous, breathable membranes for physical isolation, but these membranes often... Insufficient density, uniformity, or compatibility with micro / nano processes make it difficult to ensure the rapid passage of water vapor molecules while providing effective protection, thus affecting the sensor's response speed and accuracy. While existing technologies employ improvements such as noble metal electrodes or passivation layers deposited on electrodes to enhance electrode reliability, the sensor's detection accuracy and long-term stability in high-humidity environments remain insufficient due to the direct contact between the moisture-sensing membrane and the environment. Adding a filter membrane to the sensor's packaging port provides some protection, but long-term operation still risks membrane clogging due to dust or membrane damage. Furthermore, this approach cannot fundamentally address the accuracy degradation caused by condensation on the moisture-sensing membrane surface in high-humidity environments, leaving the sensor's long-term stability unresolved.

[0004] To achieve the above objectives, the present invention provides the following technical solution: a planar interdigitated capacitive humidity sensor with a double-layer composite film structure, comprising a sensor body, The sensor body includes a substrate, an insulating layer, a pair of planar interdigitated electrodes, an electrode passivation layer, a humidity-sensitive polymer film layer, and a protective film layer. The insulating layer is located above the substrate, the planar interdigitated electrode pair is located above the insulating layer, the electrode passivation layer is located above and to the side of the planar interdigitated electrode pair, preventing direct contact between the planar interdigitated electrode pair and the external environment, the moisture-sensitive polymer film is located above the electrode passivation layer, and the protective film is located above the moisture-sensitive polymer film. The top end of the substrate is connected to the bottom end of the insulating layer, the top end of the insulating layer is connected to the bottom end of the planar interdigitated electrode pair, the top end of the planar interdigitated electrode pair is connected to the bottom end of the electrode passivation layer, the top end of the electrode passivation layer is connected to the bottom end of the moisture-sensitive polymer film, and the top end of the moisture-sensitive polymer film is connected to the bottom end of the protective film.

[0005] As a further technical solution of the present invention, the insulating layer is one or more of silicon dioxide, borosilicate glass and fluorine-doped borosilicate glass. The insulating layer is formed on the substrate and is prepared using low dielectric constant materials such as silicon dioxide, borosilicate glass and fluorine-doped borosilicate glass to reduce the parasitic capacitance of the sensor.

[0006] As a further technical solution of the present invention, the thickness of the planar interdigitated electrode pair is 1-2 μm. The planar interdigitated electrode pair is formed on the insulating layer by micro-nano processing technology, such as magnetron sputtering, photolithography, and plasma etching. The electrode material is preferably aluminum with a thickness of 1-2 μm, which constitutes the two poles of the sensing capacitor.

[0007] As a further technical solution of the present invention, the planar interdigitated electrode pair is made of aluminum material.

[0008] As a further technical solution of the present invention, the electrode passivation layer is made of silicon nitride material, the thickness of the electrode passivation layer is 50-100nm, the electrode passivation layer covers the planar interdigitated electrode pair and part of the insulating layer surface, and is used to protect the electrode from environmental corrosion. The electrode passivation layer is preferably made of silicon nitride with excellent step coverage capability, and is prepared by low-pressure chemical vapor deposition process, with a thickness of 50-100nm.

[0009] As a further technical solution of the present invention, the humidity-sensitive polymer film is one or more of polyimide and cellulose acetate, the thickness of the humidity-sensitive polymer film is 2-3 μm, the humidity-sensitive polymer film is coated on the electrode passivation layer and covers the entire sensing area, the film is composed of a humidity-sensitive polymer material, such as polyimide and cellulose acetate, and has a thickness of 2-3 μm, serving as the core humidity-sensing medium of the sensor.

[0010] As a further technical solution of the present invention, the thickness of the protective film layer is 50-100nm. The protective film layer is formed on the moisture-sensitive polymer film layer by a vapor deposition process. The film layer material is a dense, hydrophobic, breathable and chemically stable polymer material, preferably a parylene-based material. The film layer is extremely thin, only 50-100nm, allowing water molecules to diffuse freely, but effectively blocking liquid water droplets, dust particles and most pollutants from directly contacting the underlying moisture-sensitive film. The protective film layer has excellent conformal deposition characteristics and nanoscale pore filling ability, completely covering the surface of the underlying moisture-sensitive film layer with microscopic undulations without dead corners, forming a continuous and defect-free protective barrier.

[0011] A method for fabricating a planar interdigitated capacitive humidity sensor with a double-layer composite film structure includes the following steps: Step 1: Prepare an insulating layer on a clean substrate surface: Grow or deposit a low dielectric constant insulating layer on the substrate; Step 2, Fabrication of planar intercalation electrode pairs: An aluminum film is deposited on the insulating layer by magnetron sputtering, and planar intercalation electrode pairs are formed by photolithography and dry etching processes; Step 3: Electrode passivation layer preparation: A 50-100 nm thick silicon nitride passivation layer is deposited on the entire structure surface using LPCVD process; Step 4: Preparation of moisture-sensitive polymer film: A moisture-sensitive polymer solution is applied to the sensing area on the electrode passivation layer by spin coating, spraying or dipping. After patterning and curing, a moisture-sensitive polymer film with a thickness of 2-3 μm is formed. Step 5: Preparation of protective film: The device completed in Step 4 is placed in a Parylene vapor deposition system. Under vacuum, the Parylene dimer is vaporized, decomposed, and polymerized on the device surface to form a 50-100 nm thick protective film, thus completing the preparation of the sensor chip.

[0012] Compared with the prior art, the beneficial effects of the present invention are: 1. The dense Parylene protective film acts like a nano-scale filter, allowing gas to pass through while effectively blocking dust, aerosols, salt spray pollutants in the environment from directly contacting the core moisture-sensing membrane, greatly enhancing the sensor's environmental robustness and long-term stability. 2. Parylene material itself is hydrophobic and has low surface energy. In high humidity environments, this ultra-thin hydrophobic film can significantly reduce the probability and size of microdroplet formation on the surface of the moisture-sensing film, effectively suppressing the condensation effect. At the same time, it has a certain self-cleaning ability, which enables the sensor to maintain a good capacitance-humidity linear response relationship in the area close to saturated humidity, thus broadening the effective working range of the sensor and improving the measurement accuracy and long-term working stability in the high humidity range. 3. Because the Parylene membrane is extremely thin, 50-100nm and has inherent air permeability, the resistance to water molecule diffusion through this membrane is very small. Therefore, the response speed of the sensor is almost unaffected by this additional membrane, maintaining a fast response capability and measurement hysteresis characteristics comparable to sensors without a protective layer. 4. Parylene uses room temperature vapor deposition, which has a low process temperature and will not damage the moisture-sensitive polymer film and circuitry already formed underneath. Its excellent step coverage capability ensures the formation of a uniform and non-porous protective layer at the steps of complex microstructures and interdigitated electrodes. It also provides additional moisture-proof and corrosion-proof protection for internal aluminum electrodes and interconnect structures, extending the sensor's lifespan. 5. The total thickness of the entire double-layer film structure is only 3µm, which is compact and very suitable for on-chip integration or wafer-level packaging with CMOS readout circuits. This is conducive to the miniaturization and low-cost mass production of sensors. Attached Figure Description

[0013] Figure 1 This is a schematic diagram of the architecture of the present invention; Figure 2 This is a diagram showing the connection between the insulating layer and the planar interdigitated electrode pair of the present invention; Figure 3 This is a flowchart of the present invention.

[0014] In the figure: 1. Substrate; 2. Insulating layer; 3. Planar interdigitated electrode pair; 4. Electrode passivation layer; 5. Humidity-sensitive polymer film layer; 6. Protective film layer; 7. Sensor body. Detailed Implementation

[0015] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. 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 of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0016] Please see Figure 1-3 This invention provides a planar interdigitated capacitive humidity sensor with a double-layer composite film structure, including a sensor body 7. The sensor body 7 includes a substrate 1, an insulating layer 2, a planar interdigitated electrode pair 3, an electrode passivation layer 4, a moisture-sensitive polymer film layer 5, and a protective film layer 6; An insulating layer 2 is located above the substrate 1. A planar interdigitated electrode pair 3 is located above the insulating layer 2. An electrode passivation layer 4 is located above and to the side of the planar interdigitated electrode pair 3, preventing direct contact between the planar interdigitated electrode pair 3 and the external environment. A moisture-sensitive polymer film layer 5 is located above the electrode passivation layer 4, and a protective film layer 6 is located above the moisture-sensitive polymer film layer 5. The top end of the substrate 1 is connected to the bottom end of the insulating layer 2, the top end of the insulating layer 2 is connected to the bottom end of the planar interdigitated electrode pair 3, the top end of the planar interdigitated electrode pair 3 is connected to the bottom end of the electrode passivation layer 4, the top end of the electrode passivation layer 4 is connected to the bottom end of the moisture-sensitive polymer film layer 5, and the top end of the moisture-sensitive polymer film layer 5 is connected to the bottom end of the protective film layer 6.

[0017] The insulating layer 2 is one or more of silicon dioxide, borosilicate glass, and fluorine-doped borosilicate glass.

[0018] In use, insulating layer 2 is formed on the substrate and is made of low dielectric constant materials such as silicon dioxide, borosilicate glass, and fluorine-doped borosilicate glass to reduce the parasitic capacitance of the sensor.

[0019] The thickness of the planar interdigitated electrode pair 3 is 1-2 μm.

[0020] In use, planar interdigitated electrode pairs 3 are formed on the insulating layer 2 through micro-nano processing techniques such as magnetron sputtering, photolithography, and plasma etching. The electrode material is preferably aluminum with a thickness of 1-2 μm, forming the two poles of the sensing capacitor.

[0021] The planar interdigitated electrode pair 3 is made of aluminum.

[0022] The electrode passivation layer 4 is made of silicon nitride material, and the thickness of the electrode passivation layer 4 is 50-100nm.

[0023] In use, the electrode passivation layer 4 covers the planar interdigitated electrode pair 3 and part of the insulating layer 2 to protect the electrode from environmental corrosion. The electrode passivation layer 4 is preferably made of silicon nitride with excellent step coverage capability and is prepared by low-pressure chemical vapor deposition process with a thickness of 50-100nm.

[0024] The moisture-sensitive polymer membrane layer 5 is one or more of polyimide and cellulose acetate, and the thickness of the moisture-sensitive polymer membrane layer 5 is 2-3 μm.

[0025] In use, the humidity-sensitive polymer film layer 5 is coated on the electrode passivation layer 4, covering the entire sensing area. This film layer is made of humidity-sensitive polymer materials, such as polyimide and cellulose acetate, with a thickness of 2-3 μm, and serves as the core humidity-sensing medium of the sensor.

[0026] The thickness of the protective film layer 6 is 50-100nm.

[0027] In use, the protective film layer 6 is formed on the moisture-sensitive polymer film layer 5 through a vapor deposition process. The film layer material is a dense, hydrophobic, breathable and chemically stable polymer material, preferably a parylene-based material. The film layer is extremely thin, only 50-100 nm thick, allowing water molecules to diffuse freely through, but effectively blocking liquid water droplets, dust particles and most pollutants from directly contacting the underlying moisture-sensitive film. The protective film layer 6 has excellent conformal deposition characteristics and nanoscale pore filling ability, completely covering the surface of the underlying moisture-sensitive film with micro-undulations without dead corners, forming a continuous and defect-free protective barrier.

[0028] A method for fabricating a planar interdigitated capacitive humidity sensor with a double-layer composite film structure includes the following steps: Includes the following steps: Step 1: Prepare insulating layer 2 on clean substrate 1 surface: Grow or deposit low dielectric constant insulating layer 2 on substrate 1; Step 2, Fabrication of planar intercalation electrode pair 3: An aluminum film is deposited on the insulating layer 2 by magnetron sputtering, and the planar intercalation electrode pair 3 is formed by photolithography and dry etching processes; Step 3, Electrode passivation layer 4 preparation: A 50-100 nm thick silicon nitride passivation layer is deposited on the entire structure surface using LPCVD process; Step 4: Preparation of moisture-sensitive polymer film 5: A moisture-sensitive polymer solution is applied to the sensing area on the electrode passivation layer 4 by spin coating, spraying or dipping. After patterning and curing, a 2-3 μm thick moisture-sensitive polymer film 5 is formed. Step 5, Preparation of protective film layer 6: The device completed in step 4 is placed in the Parylene vapor deposition system. Under vacuum, the Parylene dimer is vaporized, decomposed, and polymerized on the device surface to form a 50-100nm thick protective film layer 6, thus completing the preparation of the sensor chip.

[0029] Example 1: In this invention, a silicon substrate 1 is formed with a 500 nm thick silicon dioxide insulating layer 2 by thermal oxidation; a 1.5 μm thick planar interdigitated electrode pair 3 is formed on the insulating layer 2 using magnetron sputtering, photolithography, and reactive ion etching processes; a 100 nm thick silicon nitride electrode passivation layer 4 is deposited on the electrode structure using LPCVD; a 2.5 μm thick moisture-sensitive polymer film 5 is formed by spin-coating a polyimide precursor solution and undergoing stepwise curing, such as pre-curing at 120°C and final curing at 250°C; finally, a 150 nm thick Parylene film is deposited in a dedicated Parylene deposition apparatus using the Gorham process. H-type is used as the protective film layer 6. Tests show that the sensor exhibits good linear capacitance response in the range of 10%-90%RH, with a response time of 63% less than 5 seconds. After continuous exposure to a high humidity environment of 95%RH for 24 hours, its capacitance drift is less than the effective value of 2%RH, which is much lower than that of the control sample without the Parylene protective layer, with a drift >10%RH. At the same time, its performance did not show significant degradation in dusty environments, demonstrating excellent protective effects.

[0030] Example 2: In this invention, unlike Example 1, the material of the moisture-sensitive polymer membrane layer 5 is replaced with cellulose acetate butyrate, with a thickness of 3 μm; the protective membrane layer 6 uses Parylene F, with a deposition thickness of 200 nm; and the insulating layer 2 uses FBSG to further reduce parasitic capacitance. This sensor also exhibits fast response, high linearity, and excellent high-humidity stability. The waterproof and breathable protective membrane layer 6 is made of Parylene material, with a thickness of 50-100 nm, and combines the characteristics of being dense, hydrophobic, and breathable. It allows water molecules to pass through quickly, ensuring a fast response speed, while effectively blocking liquid water droplets, dust contaminants, and contact with the moisture-sensitive membrane, and significantly suppressing surface condensation in high-humidity environments. This membrane layer effectively alleviates the industry problem of nonlinear degradation of response characteristic curves and decreased stability of moisture-sensitive materials in high-humidity environments due to surface condensation and swelling. Through this innovative double-layer membrane structure, this invention significantly improves the detection accuracy, response linearity, long-term stability, and environmental adaptability of the sensor in high-humidity environments, and is widely applicable to meteorological monitoring, consumer electronics, automobiles, industrial humidity control, and medical equipment.

[0031] 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 planar interdigitated capacitive humidity sensor with a double-layer composite film structure, comprising a sensor body (7). Its features are: The sensor body (7) includes a substrate (1), an insulating layer (2), a planar interdigitated electrode pair (3), an electrode passivation layer (4), a humidity-sensitive polymer film layer (5), and a protective film layer (6). The insulating layer (2) is located above the substrate (1), the planar interdigitated electrode pair (3) is located above the insulating layer (2), the electrode passivation layer (4) is located above and to the side of the planar interdigitated electrode pair (3), preventing the planar interdigitated electrode pair (3) from directly contacting the external environment, the moisture-sensitive polymer film layer (5) is located above the electrode passivation layer (4), and the protective film layer (6) is located above the moisture-sensitive polymer film layer (5). The top end of the substrate (1) is connected to the bottom end of the insulating layer (2), the top end of the insulating layer (2) is connected to the bottom end of the planar interdigitated electrode pair (3), the top end of the planar interdigitated electrode pair (3) is connected to the bottom end of the electrode passivation layer (4), the top end of the electrode passivation layer (4) is connected to the bottom end of the moisture-sensitive polymer film layer (5), and the top end of the moisture-sensitive polymer film layer (5) is connected to the bottom end of the protective film layer (6).

2. The planar interdigitated capacitive humidity sensor with a double-layer composite film structure according to claim 1, characterized in that: The insulating layer (2) is one or more of silicon dioxide, borosilicate glass and fluorine-doped borosilicate glass.

3. The planar interdigitated capacitive humidity sensor with a double-layer composite film structure according to claim 1, characterized in that: The thickness of the planar interdigitated electrode pair (3) is 1-2 μm.

4. A planar interdigitated capacitive humidity sensor with a double-layer composite film structure according to claim 1, characterized in that: The planar interdigitated electrode pair (3) is made of aluminum, a non-precious metal.

5. A planar interdigitated capacitive humidity sensor with a double-layer composite film structure according to claim 1, characterized in that: The electrode passivation layer (4) is made of silicon nitride material and has a thickness of 50-100 nm.

6. A planar interdigitated capacitive humidity sensor with a double-layer composite film structure according to claim 1, characterized in that: The moisture-sensitive polymer membrane (5) is one or more of polyimide and cellulose acetate, and the thickness of the moisture-sensitive polymer membrane (5) is 2-3 μm.

7. A planar interdigitated capacitive humidity sensor with a double-layer composite film structure according to claim 1, characterized in that: The thickness of the protective film layer (6) is 50-100nm.

8. A method for preparing a planar interdigitated capacitive humidity sensor with a double-layer composite film structure according to any one of claims 1-7, characterized in that, Includes the following steps: Step 1: Prepare an insulating layer (2) on a clean substrate (1) surface: Grow or deposit a low dielectric constant insulating layer (2) on the substrate (1). Step 2, Preparation of planar intercalation electrode pair (3): An aluminum film is deposited on the insulating layer (2) by magnetron sputtering, and a planar intercalation electrode pair (3) is formed by photolithography and dry etching processes. Step 3, Electrode passivation layer (4) preparation: A 50-100 nm thick silicon nitride passivation layer is deposited on the entire structure surface using LPCVD process; Step 4, Preparation of moisture-sensitive polymer film (5): On the sensing area of ​​the electrode passivation layer (4), a moisture-sensitive polymer solution is applied by spin coating, spraying or dipping, and after patterning and curing, a moisture-sensitive polymer film (5) with a thickness of 2-3 μm is formed. Step 5, Preparation of protective film (6): Place the device that has completed step 4 into the Parylene vapor deposition system. Under vacuum, the Parylene dimer is vaporized, decomposed and polymerized on the surface of the device to form a 50-100nm thick protective film (6), thus completing the preparation of the sensor chip.

Images