Dual-mode flexible calorimetric flow sensor based on paper-cut structure and preparation method thereof

Abstract

The application belongs to the technical field of sensors, and particularly relates to a double-mode flexible calorimetric flow sensor based on a paper-cut structure and a preparation method thereof. The first flexible substrate layer, the second flexible substrate layer, the first thermistor, the second thermistor and the third thermistor are sequentially formed on a silicon wafer. Different from a silicon-based processing technology, the application first proposes a flexible substrate suspension structure integrated processing technology. A sacrificial layer is first constructed at a cavity, and after the construction of a sensing layer is completed, the application adopts a process of washing the thick glue of the sacrificial layer with acetone to form a suspension structure, which is different from directly etching the substrate on the silicon base. Then, the silicon substrate is peeled off to form a flexible calorimetric flow sensor with a suspension structure. Since the application adopts the integrated processing technology and the double-mode flexible substrate to manufacture the sensor and the suspension and paper-cut structure thereof, the sensor has the characteristics of high stability, low heat dissipation, high sensitivity and complex curved surface shape retention.

Description

Technical Field

[0001] This invention belongs to the field of sensor technology, specifically relating to a dual-mode flexible calorimetric flow sensor based on a paper-cut structure. This invention also relates to a method for preparing a dual-mode flexible calorimetric flow sensor based on a paper-cut structure. Background Technology

[0002] Respiratory monitoring instruments are important devices for monitoring the respiratory health status of humans.

[0003] Existing respiratory monitoring devices are mostly bulky, expensive, poorly portable, and require closed-back breathing masks, making them unsuitable for monitoring the respiratory health of non-critical patients. In recent years, the deep integration of the Internet of Things and flexible electronics has driven the rapid development of respiratory monitoring technology. Real-time monitoring of the respiratory health of patients with respiratory diseases using small, low-power, low-cost, and flexible wearable respiratory sensors has become an inevitable trend.

[0004] Currently, there are many types of flexible sensors that can characterize human respiration, and scholars both domestically and internationally have conducted in-depth research on them, mainly focusing on sensor materials, sensitivity, response time, and flexible microstructures, achieving certain results. Most reported respiratory sensors monitor human respiration through mechanisms such as chest and abdominal vibration, changes in respiratory airflow temperature, and changes in respiratory airflow humidity. These sensors can only obtain respiratory frequency information and cannot effectively measure more medically diagnostic information such as respiratory airflow velocity, airflow direction, and ventilation volume, severely limiting the practical application of flexible respiratory sensors. Flow sensors have inherent advantages in this regard, among which calorimetric flow sensors are more widely studied due to their simple fabrication methods and high stability.

[0005] Existing calorimetric flexible flow sensors are designed and fabricated directly on flexible substrates, which has significant drawbacks: First, the flow sensors use a thermal sensing principle, and fabricating directly on a flexible substrate results in substantial heat dissipation, making it difficult to achieve high sensitivity and response speed. This is a key problem facing the application of calorimetric flexible flow sensors in respiratory monitoring.

[0006] To address the aforementioned issues, the suspended structure fabricated for silicon-based calorimetric flow sensors, designed to mitigate high heat dissipation, offers a promising solution. This involves incorporating the suspended structure into calorimetric flow sensors based on flexible substrates. However, the fabrication methods for the suspended structure in silicon-based calorimetric flow sensors are limited to deep reactive ion etching (DRIE) and the silicon substrate beneath the selenium fluoride release sensor, or wet etching of the silicon substrate.

[0007] However, the fabrication of silicon-based suspended structures is based on the anisotropy or isotropy of different crystal orientations of the silicon substrate, while flexible materials do not possess this property. Therefore, this method is not suitable for flexible substrates, especially since the design of the crucial suspended layer cannot be achieved through etching.

[0008] Secondly, flexible substrates are planar structures, making it impossible to achieve conformal fitting of complex curved surfaces, leading to easy sensor detachment and deformation. Regarding sensor placement, Gauss's theorem states that the Gaussian curvature of a surface remains consistent under local isometric transformations, thus geometrically, planar substrates cannot fit into non-stretchable curved surfaces. Reported flexible flow sensors are all designed on non-stretchable planar flexible substrates, failing to achieve conformal fitting when applied to human body parts, easily resulting in wrinkles, deformation, and detachment. Expansion structures, due to their unique geometric properties (negative Poisson's ratio), can achieve conformal fitting of complex curved surfaces. Such structures only require outlining simple slit patterns on a two-dimensional substrate to obtain different cutting unit matrices (including rectangles, triangles, hexagons, stars, and even their hybrid tessellation), achieving the stretchability of planar materials. Therefore, leveraging the research results on expansion structures, developing flexible substrates and structures with conformal fitting capabilities for complex curved surfaces, endowing flexible sensors with stretchability, and enabling conformal fitting of respiratory sensors to human monitoring areas (such as the bridge of the nose and cheeks) is a crucial problem that urgently needs to be solved. Summary of the Invention

[0009] The main objective of this invention is to provide a dual-mode flexible calorimetric flow sensor and its fabrication process. This overcomes the shortcomings of existing technologies to meet the needs of flow sensors in fields such as smart healthcare and health monitoring. The sensor utilizes a PDMS-Parylene C dual-mode flexible substrate, endowing the flexible substrate bonding surface with ultra-soft properties and the sensing film layer with high strength and high stability. The suspended structure provides low power consumption and high sensitivity, while the substrate's paper-cut structure facilitates easy shape-fitting.

[0010] The objective of this invention is achieved through the following technical solution.

[0011] This invention discloses a dual-mode flexible calorimetric flow sensor, comprising a first flexible substrate layer and a second flexible substrate layer, wherein the second flexible substrate layer is disposed above the first flexible substrate layer, and the second flexible substrate layer includes a first thermistor disposed in the center, and the first flexible substrate layer includes a second thermistor and a third thermistor disposed on both sides of the first thermistor.

[0012] As a preferred embodiment, the first flexible substrate layer is a PDMS film located at the bottom of the sensor.

[0013] As a preferred embodiment, the second flexible substrate layer is a Parylene C film.

[0014] As a preferred embodiment, the first thermistor is made of platinum.

[0015] As a preferred embodiment, both the second thermistor and the third thermistor are made of platinum.

[0016] This invention also provides a method for fabricating a dual-mode flexible calorimetric flow sensor, comprising the following steps:

[0017] Step 1: Prepare a silicon wafer and clean it;

[0018] Step 2: Spin-coat a first flexible substrate layer onto the silicon wafer;

[0019] Step 3: Spin-coat a layer of photoresist onto the first flexible substrate layer, and perform photolithographic patterning using the first mask;

[0020] Step 4: Spin-coat a layer of the same material as the first flexible substrate layer;

[0021] Step 5: Polish the surface to make the sacrificial layer thick adhesive flush with the first flexible substrate layer;

[0022] Step 6: Deposit a second flexible substrate layer;

[0023] Step 7: Spin-coat a layer of photoresist and use a second mask to perform photolithographic patterning;

[0024] Step 8: Vacuum sputtering of the metal layer, followed by immersion in acetone for stripping;

[0025] Step 9: Vacuum deposition of a second flexible substrate layer;

[0026] Step 10: Pattern the photoresist with the second mask and etch the second flexible substrate layer to form the window of the paper-cut two-dimensional expansion structure and the etch window of the sacrificial layer;

[0027] Step 11: Release the sacrificial layer thick adhesive to form a suspension structure;

[0028] Step 12: Peel off the silicon substrate to form the entire sensor.

[0029] As a preferred option, before performing step 2, the silicon surface is pretreated with the coupling agent trichlorosilane to reduce the adhesion strength between the first flexible substrate layer and the silicon surface, making the separation of the flexible film and the silicon wafer more efficient and safer.

[0030] As a preferred option, the metal selected in step 8 is platinum, which has a large temperature coefficient of thermal sensitivity and low resistivity.

[0031] Beneficial effects: Utilizing an integrated processing technology and a paper-cut structure, the sensor exhibits excellent flexibility, sensitivity, stability, low heat dissipation, and fast response. This effectively solves the problems of high heat dissipation, relatively limited sensitivity and response speed, and easy detachment and deformation of the planar substrate in current flow sensors. Furthermore, the manufacturing process is simple, facilitating large-scale production from an industrialization perspective. Attached Figure Description

[0032] The present invention will be further described below with reference to the accompanying drawings and embodiments:

[0033] Figure 1 This is a flowchart illustrating the fabrication process of the dual-mode flexible calorimetric flow sensor of the present invention.

[0034] Figure 2 This is a three-dimensional structural schematic diagram of the dual-mode flexible calorimetric flow sensor of the present invention;

[0035] Figure 3 This is a cross-sectional view of the dual-mode flexible calorimetric flow sensor of the present invention;

[0036] Figure 4 This is the detection circuit structure of the dual-mode flexible calorimetric flow sensor of the present invention;

[0037] Figure 5 This is a schematic diagram of the expansion structure of the paper-cut bridge connection under full stretch.

[0038] Figure 6 This is a diagram showing the effect of conformal bonding of a spherical surface.

[0039] In the figure: 1-First flexible substrate layer; 2-Second flexible substrate layer; 3-First thermistor; 4-Second thermistor; 5-Third thermistor. Detailed Implementation

[0040] To better illustrate the purpose and advantages of the present invention, the invention will be further described below in conjunction with the accompanying drawings and examples.

[0041] See Figures 2 to 3 As shown: This embodiment discloses a dual-mode flexible calorimetric flow sensor including a first flexible substrate layer 1, a groove structure and an island chain bridging paper-cut structure disposed in the center of the first flexible substrate layer 1, a second flexible substrate layer 2 disposed on the first flexible substrate layer 1, a first thermistor 3 disposed in the center of the second flexible substrate layer 2, and a second thermistor 4 and a third thermistor 5 disposed on both sides of the first thermistor 3.

[0042] The substrate material can be a silicon wafer.

[0043] The substrate material can be polydimethylsiloxane (PDMS) or polydichloro-[2,2]-paracyclophane.

[0044] The chemical state of polydimethylsiloxane is dimethyl silicone oil, a colorless or pale yellow liquid, odorless, with high transparency. It possesses heat resistance, cold resistance, minimal viscosity change with temperature, water resistance, low surface tension, thermal conductivity, and light transmittance. Dimethyl silicone oil is non-toxic and odorless, physiologically inert, and has good chemical stability. It exhibits good electrical insulation, weather resistance, hydrophobicity, and high shear strength, allowing for long-term use at temperatures ranging from -50℃ to 200℃. It also possesses excellent physical properties.

[0045] Poly(p-dichlorotoluene) is a protective polymer material that can be vapor-deposited under vacuum. Its molecular penetrating power allows it to form a high-quality, pinhole-free protective layer inside, on the bottom, and around components.

[0046] See Figure 1 As shown: This invention also provides a method for fabricating a dual-mode flexible calorimetric flow sensor. Figure 1 This is a flowchart illustrating the fabrication process of the flexible sensor in an embodiment of the present invention, including the following steps:

[0047] Step 1: Prepare a silicon wafer and clean it; for example: vacuum deposit 10 microliters of FAS on a 4-inch single-sided polished silicon wafer.

[0048] Step 2: Spin-coat a first flexible substrate layer 1 onto the silicon wafer. Before spin-coating the first flexible substrate layer 1, pre-treat the silicon surface with a coupling agent, trichlorosilane, to reduce the adhesion strength between the first flexible substrate layer 1 and the silicon surface, making the separation of the flexible film and the silicon wafer more efficient and safer. For example, a spin coater operates at 1000 rpm for 30 seconds to spin-coat PDMS (PDMS:curing agent = 10:1) onto the wafer surface, with a PDMS thickness of 150 μm.

[0049] Step 3: Spin-coat a layer of photoresist and use the first mask for photolithographic patterning; for example: spin-coat 17µm photoresist AZ9260 at 1000rpm for 30 seconds, bake for 4.5 minutes, then develop and pattern the photoresist AZ9260. Expose for approximately 70 seconds, with a developer ratio of AZ400k:water = 1:3, and the development process takes 5 minutes.

[0050] Step 4: Spin coat a layer of the same material as the first flexible substrate 1. For example, spin coater operates at 1000 rpm for 30 seconds to spin coat PDMS (PDMS: curing agent = 10:1) onto the wafer surface. The PDMS thickness is 150 μm.

[0051] Step 5: Polish the sacrificial layer to make it flush with the first flexible substrate 1. For example, polish the top PDMS layer using chemical mechanical polishing (CMP) or reactive ion etching (RIE) until the patterned photoresist is exposed.

[0052] Step 6: Deposit a second flexible substrate layer 2, for example: a parylene-c film of about 10 μm by chemical vapor deposition (CVD).

[0053] Step 7: Spin-coat a layer of photoresist (type of photoresist), and use a second mask to perform photolithographic patterning; for example: spin-coat AZ506 photoresist (4000rpm x 30 seconds, soft bake for 1 minute, about 2um) and pattern (exposure for 6.5 seconds, FHD-5 development for 1 minute).

[0054] Step 8: Vacuum sputtering of a metal layer, followed by immersion in acetone for peeling; the metal chosen is platinum, which has a high temperature coefficient of thermal susceptibility and low resistivity. For example: vacuum sputtering a metal layer (50nm TiW + 100nm Pt), followed by immersion in acetone for peeling (or ultrasonic treatment).

[0055] Step 9: Vacuum deposit a second flexible substrate 2, for example: chemical vapor deposition (CVD) of a parylene-c film of about 10 μm.

[0056] Step 10: Pattern the photoresist using the second mask and etch the second flexible substrate 2 to form the window of the paper-cut two-dimensional expansion structure and the etching window of the sacrificial layer. For example: spin coat 17µm photoresist AZ9260 at 1000rpm for 30 seconds, bake for 4.5 minutes, then develop and pattern the photoresist AZ9260. Expose for about 70 seconds, with a developer ratio of AZ400k:water = 1:3, and the development process takes 5 minutes. Also etch excess parylene-c and pdms (RIE).

[0057] Step 11: Release the sacrificial layer photoresist to form a suspension structure, resulting in the structure shown in the figure. For example: Dissolve the sacrificial layer photoresist by immersing it in acetone through the etching window.

[0058] Step 12: Peel off the silicon substrate to form the entire sensor.

[0059] This invention forms a calorimetric flow sensor by sequentially forming a first flexible substrate layer 1, a second flexible substrate layer 2, a first thermistor 3, a second thermistor 4, and a third thermistor 5 on a silicon wafer, along with a paper-cut structure and a suspended structure based on a dual-mode substrate. Unlike silicon-based fabrication processes, this invention innovatively proposes an integrated fabrication process for the suspended structure on a flexible substrate. First, a sacrificial layer is constructed in the cavity. After the sensing layer is constructed, unlike direct etching of the silicon substrate, this invention uses acetone to wash away the thick adhesive of the sacrificial layer to form the suspended structure. Then, the silicon substrate is peeled off to form a flexible calorimetric flow sensor with a suspended structure. Because this invention uses an integrated fabrication process and a dual-mode flexible substrate to fabricate the sensor and its suspended and paper-cut structures, the sensor exhibits high stability, low heat dissipation, high sensitivity, and conformal bonding to complex curved surfaces.

[0060] See Figure 4 As shown: This invention illustrates the detection circuit structure of a dual-mode flexible calorimetric flow sensor, where Ru is a first thermistor; Rd is a second thermistor; R1 and R2 are fixed resistors; Rp is an adjustable resistor; and Op is a differential amplifier. When fluid flows through the sensor, the resistance change data or the amplified voltage data is used as the signal transmission data.

[0061] Figure 5 and Figure 6 These are schematic diagrams of the paper-cut bridge connection expansion structure and spherical conformal bonding effect of the dual-mode flexible calorimetric flow sensor under full stretching state. The practical application of the dual-mode flexible calorimetric flow sensor provided by this invention can include: attaching the sensor to the human body, leading out a wire from one end, connecting to an external signal processing module, and measuring data such as the speed of respiratory airflow when the person breathes.

[0062] In this invention, the resistance or voltage value is related to the fluid flow rate, and the signal response is determined by the change in the resistance or voltage value.

[0063] On the one hand, the integrated processing technology based on a flexible substrate suspension structure provided by this invention enables the sensor to have excellent sensitivity, stability, low heat dissipation, and fast response, which can effectively solve the problems of large heat dissipation and relatively limited sensitivity and response speed of current flow sensors. Moreover, the manufacturing process is simple and facilitates large-scale production from an industrialization perspective.

[0064] On the one hand, the processing methods for silicon-based suspended structures are not suitable for flexible substrates. On the other hand, it should be noted that achieving conformal bonding to non-developable surfaces (surfaces with non-zero Gaussian curvature, such as spheres) is an important goal in developing curved electronic products. The Ingenious Theorem states that to achieve this goal, planar electronic products must possess the characteristic of local non-equidistant transformation, i.e., stretchability. Paper-cut island-bridge linked expansion structures, due to their single-degree-of-freedom stretching and uniform deformation characteristics, have already been applied in fields such as curved optoelectronic devices. However, traditional two-dimensional expansion structures composed of rotating units are often damaged due to the huge strain at rigid nodes, which greatly increases the difficulty of circuit laying. When bonding to curved surfaces, the stress at the nodes further increases, leading to structural damage or poor bonding. Furthermore, planar wire cutting can only produce stretchable expansion structures, while covering complex curved surfaces requires electronic devices to possess both stretchability and compressibility. Inspired by paper-cut structures, this patent proposes to manufacture an adaptively bonding respiration detector by replacing rigid nodes with paper-cut structure island-bridge links to connect rotating units and combining them with a suspended flow sensor.

[0065] On the other hand, this invention proposes to manufacture an adaptive breathing detector by replacing rigid node connections with island-bridge links of a paper-cut structure and combining them with a suspended flow sensor. The island-bridge links overcome the stress concentration problem at rigid nodes. The flexible connecting bridges can bend to accommodate a tighter fit of the device to curved surfaces, and their folded neutral positions allow for expansion and compression of the structure, increasing its durability. The flow sensor exhibits conformal fit and good flexibility.

[0066] The above are merely preferred embodiments of this application and are not intended to limit this application. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of this application should be included within the scope of protection of this application. Other structures and principles are the same as those in the prior art and will not be described in detail here.

Claims

1. A method for fabricating a dual-mode flexible calorimetric flow sensor based on a paper-cutting structure, characterized in that: Includes the following steps: Step 1: Prepare a silicon wafer and clean it; Step 2: Spin-coat a first flexible substrate layer onto the silicon wafer; Step 3: Spin-coat a layer of photoresist onto the first flexible substrate layer, and perform photolithographic patterning using the first mask; Step 4: Spin-coat a layer of the same material as the first flexible substrate layer; Step 5: Polish the surface to make the sacrificial layer thick adhesive flush with the first flexible substrate layer; Step 6: Deposit a second flexible substrate layer; Step 7: Spin-coat a layer of photoresist and use a second mask to perform photolithographic patterning; Step 8: Vacuum sputtering of the metal layer, followed by immersion in acetone for stripping; platinum was selected as the metal. Step 9: Vacuum deposition of a second flexible substrate layer; Step 10: Pattern the photoresist with the second mask and etch the second flexible substrate layer to form the window of the paper-cut two-dimensional expansion structure and the etch window of the sacrificial layer; Step 11: Release the sacrificial layer thick adhesive to form a suspension structure; Step 12: Peel off the silicon wafer to form the entire sensor; A calorimetric flow sensor is formed by sequentially forming a first flexible substrate layer, a second flexible substrate layer, a first thermistor, a second thermistor, and a third thermistor on a silicon wafer, as well as a paper-cut structure and a suspended structure based on a dual-mode substrate. The second flexible substrate layer includes a first thermistor disposed in the center, and the first flexible substrate layer includes a second thermistor and a third thermistor disposed on both sides of the first thermistor.

2. The method for fabricating a dual-mode flexible calorimetric flow sensor as described in claim 1, characterized in that: Before performing step 2, the silicon surface is pretreated with the coupling agent trichlorosilane to reduce the adhesion strength between the first flexible substrate and the silicon surface, making the separation of the first flexible substrate and the silicon wafer more efficient and safer.

3. The method for fabricating a dual-mode flexible calorimetric flow sensor as described in claim 1, characterized in that: The metal selected in step 8 is platinum, which has a large temperature coefficient of thermal sensitivity and low resistivity.

4. A dual-mode flexible calorimetric flow sensor based on a paper-cutting structure, fabricated using the method described in claim 1, characterized in that: It includes a first flexible substrate layer and a second flexible substrate layer, wherein the second flexible substrate layer is disposed above the first flexible substrate layer.

5. The dual-mode flexible calorimetric flow sensor as described in claim 4, characterized in that: The first flexible substrate layer is a PDMS film located at the bottom of the sensor.

6. The dual-mode flexible calorimetric flow sensor as described in claim 4, characterized in that: The second flexible substrate layer is a Parylene C film.

7. The dual-mode flexible calorimetric flow sensor as described in claim 4, characterized in that: The first thermistor is made of platinum.

8. The dual-mode flexible calorimetric flow sensor as described in claim 4, characterized in that: Both the second and third thermistors are made of platinum metal and have a serpentine structure.

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