A fully PI laser-induced graphene switch-triggered flexible uniaxial accelerometer and its fabrication method

Abstract

This application discloses a fully PI laser-induced graphene switch-triggered flexible single-axis accelerometer and its fabrication method. The accelerometer includes a flexible top cover, a flexible structural layer, and a flexible base plate. A hexagonal central heat source mass block, suspended by a honeycomb support structure, is located at the center of the flexible structural layer. A laser-induced graphene central heat source resistor is mounted on the mass block, and laser-induced graphene strip thermistors are symmetrically arranged on both sides. Metal electrodes are provided at both ends of the central heat source resistor and the thermistors. The thermistors detect changes in the thermal field under the input acceleration along the plane of the structural layer and output an electrical signal. This electrical signal can be used to trigger a switch output through threshold comparison. The flexible top cover and flexible base plate are formed with grooves using laser cutting and then encapsulated to form a cavity. This invention has advantages such as good fully flexible adaptability, structural stability, impact resistance, simple processing, low cost, and ease of mass production. It can be widely used in flexible electronics, wearable devices, medical devices, and other fields.

Description

Technical Field

[0001] This invention relates to a technical field of detecting acceleration and attitude parameters of a moving carrier by utilizing the mechanical compliance of a PI (polyimide) substrate and the thermosensitive properties of laser-ablated graphene resistors. In particular, it relates to a fully PI laser-induced graphene flexible switch-triggered single-axis accelerometer and its fabrication method, belonging to the field of inertial measurement. Background Technology

[0002] With the rapid development of flexible electronics and wearable technology, emerging scenarios such as logistics parcel collision monitoring, wearable motion triggering, smart home flexible panel control, and medical device human motion triggering have placed new demands on accelerometers. In these scenarios, the current application status of silicon-based rigid accelerometers is as follows: (1) Logistics package collision monitoring: The sensor needs to fit the irregular package surface and be able to withstand high-frequency vibration and impact during transportation. However, the rigid structure of silicon-based devices is prone to breakage due to extrusion and the cost is high (≥5 USD per set), making it difficult to achieve disposable use. (2) Wearable motion triggering: requires the sensor to be thin, flexible and biocompatible. The rigid substrate of silicon-based devices will cause a skin-grinding sensation and cannot bend with human movement, making them easy to fall off or be damaged. (3) Smart home flexible panel trigger: It needs to be integrated with the curved panel. The rigidity of silicon-based devices destroys the integrity of the panel and requires additional holes for installation, which affects the product appearance and user experience. (4) Triggering of human movement in medical devices: Implantable or surface-adhesive devices require sensors to be ultra-thin (thickness <500μm), soft and non-invasive. The hardness and volume of silicon-based devices are difficult to meet the requirements of biocompatibility and implantation.

[0003] While silicon-based rigid accelerometers perform well in traditional applications, their inherent limitations have become a technological bottleneck in the aforementioned emerging flexible applications. (1) Poor form adaptability: Silicon-based devices rely on planar mounting and cannot fit non-planar carriers such as curved surfaces, fabrics, and human skin. Forced mounting will cause signal drift (error ≥10%) or structural damage. (2) Insufficient reliability: Silicon-based MEMS structures are brittle, and in dynamic scenarios such as logistics collisions and moving and pulling, the failure rate of sensitive structures is as high as 30% or more. (3) Cost and power consumption contradiction: The photolithography and packaging process of silicon-based devices is complex, and the cost of a single set is 5-10 times that of flexible devices. In addition, continuous high-precision measurement leads to high power consumption (static power consumption ≥100μA), which cannot be adapted to low-power scenarios such as disposable labels and micro medical devices. (4) Redundant Functional Adaptation: Switch-type triggering scenarios (such as motion start-up, collision alarm) only need to determine whether "acceleration exceeds the threshold". The continuous high-precision measurement function of silicon-based devices is redundant, which increases the complexity of signal processing. In traditional accelerometers, heat flow accelerometers sense the temperature field formed by the heater through symmetrically distributed thermistors in a sealed cavity. The temperature difference caused by the asymmetry of the temperature field due to acceleration is converted into a voltage signal that is linearly related to the magnitude of acceleration and whose direction is determined by the polarity of the temperature difference through a Wheatstone bridge. Because there are no moving parts and the sensitive mass is gas, in addition to having the characteristics of simple structure, impact resistance and low cost, its sensing principle is suitable for application in flexible inertial sensors, with strong flexibility. Chinese Patent: Omnidirectional Dynamic Heat Source Z The axial micromechanical accelerometer and its fabrication method (patent number: 202111410354.X) uses a silicon-based, wheel-like sensing structure. A circular mass (center) is suspended and kept horizontal by multiple semi-circular spokes (also called support beams). However, this support structure is unsuitable for flexible materials. Using flexible materials in this wheel-like support structure would cause the central circular mass to not be suspended, risking it touching the bottom and causing the sensing structure to fail. The sensor uses sputtered platinum to form the thermistor; however, platinum is expensive, increasing the cost of this sensor. This sensor is entirely silicon-based. This technology is unsuitable for flexible sensors that can conform to curved surfaces for detection. The sensor is composed of materials such as silicon, silicon nitride, and platinum bonded together. The mismatch in internal stress caused by these different materials, coupled with the slow release of internal stress after encapsulation, leads to significant temperature and time drift. Furthermore, the two balancing resistors of the Wheatstone bridge in the signal extraction circuit are external to the sensing element, resulting in structural incompatibility with the flexible sensor, susceptibility to signal drift due to mechanical / thermal mismatch, and complex assembly. Existing technologies often employ spiral or wavy resistors, whose complex shapes increase manufacturing difficulty. This complex structure also increases the resistor's heat capacity and reduces the sensor's response speed.

[0004] Therefore, in order to meet the application requirements of flexible inertial sensors, there is an urgent need to provide a support structure that is particularly suitable for flexible materials, and to combine it with laser ablation process to form graphene thermistors, so as to realize a flexible heat flux gyroscope with simple structure, low cost and suitable for flexible integration. Summary of the Invention

[0005] In view of the problems existing in the background technology and in response to the application requirements of flexible inertial sensors, the purpose of this invention is to provide a fully PI laser-induced graphene switch-triggered flexible single-axis accelerometer.

[0006] The technical solution adopted in this invention is: a flexible single-axis accelerometer with all-PI laser-induced graphene switch triggering, comprising a flexible top cover, a flexible structural layer, and a flexible base plate, wherein the flexible top cover, flexible structural layer, and flexible base plate are all made of polyimide (PI) flexible material; a central heat source mass block is disposed at the center of the flexible structural layer, and the central heat source mass block is suspended at the center of the structural layer by a honeycomb support structure; a central heat source resistor formed by laser-induced graphene is disposed on the central heat source mass block, and laser-induced graphene-shaped resistors are symmetrically disposed on both sides of the central heat source mass block. The thermistor is formed; the central heat source resistor is used to generate a dynamic thermal field, and the thermistor is used to detect the change of the thermal field around the central heat source mass block under the action of acceleration input along the plane of the structural layer and output an electrical signal related to the acceleration; the upper cover and the bottom plate are respectively set on the upper and lower sides of the structural layer and connected to the structural layer to form a cavity to accommodate the central heat source mass block and the thermistor; metal electrodes are provided at both ends of the central heat source resistor and the thermistor for connection to external excitation circuit and signal extraction circuit; the electrical signal can be used to achieve switch trigger output through threshold comparison.

[0007] As a further supplement to the above technical solution, a hexagonal mass block is provided at the center of the structural layer; a graphene resistor is formed on the mass block by laser ablation of PI, and after being heated by electricity, a central heat source mass block is formed.

[0008] As a further explanation of the above technical solution, the central heat source mass block is suspended at the center of the structural layer and kept horizontal by a honeycomb heat source support structure. The honeycomb heat source support structure has advantages such as light weight, relatively uniform stress distribution, and good structural stability, and is suitable for supporting and suspending the central heat source mass block under flexible material conditions.

[0009] As a further supplement to the above technical solution, the thermistor is preferably a pair of slender graphene thermistors symmetrically arranged on both sides of the central heat source mass block. Slender graphene resistors have the characteristics of small heat capacity, low thermal inertia, and fast response speed. At the same time, their longer structure helps to increase the surface area of ​​the thermistor, improve the sensitivity to temperature changes, and thus enhance the sensor sensitivity.

[0010] As a further explanation of the above technical solution, the central heat source resistor and the thermistor are both formed in situ on the PI flexible material through laser ablation process, without the need for additional sputtering of platinum metal thermistors, thereby reducing the device fabrication cost and improving the consistency of the fully flexible material system.

[0011] As a further explanation of the above technical solution, the upper cover forms grooves on the PI sheet using a laser dot matrix process, and after being encapsulated with the structural layer, a cavity is formed above the structural layer. The cavity is used to accommodate the central heat source mass block and the thermistor, and provides a relatively independent working space for the sensitive area.

[0012] As a further explanation of the above technical solution, when a linear acceleration is input along the x-axis (or y-axis), the central heat source mass block moves in the same direction as the acceleration under the action of the acceleration, causing an asymmetrical temperature field distribution generated by the hot airflow. The temperatures of two adjacent thermistors TX1 and TX2 in the same direction change in opposite directions. The thermistor biased towards the central heat source mass block has a higher temperature than the thermistor parallel to it, resulting in a temperature difference between the two adjacent thermistors TX1 and TX2. TX1 and TX2 are used as two arms of a Wheatstone bridge, and the other two arms are balanced graphene resistors R. The area of ​​R is twice that of the graphene thermistor, and its resistance is greater than or equal to TX1 or TX2. The graphene resistors R are respectively placed outside the thermistors TX1 and TX2, located outside the cavity, and are unaffected by the hot airflow. The bridge is as follows: Figure 6 As shown. The temperature difference caused by the acceleration of the input line, according to the thermal resistance effect, is converted into a change in the resistance of the bridge arms, thereby causing an unbalanced voltage in the bridge that is proportional to the input acceleration. V The linear acceleration along the x-axis can be calculated based on the output voltage, thus sensing the acceleration in the X-direction and using it to apply the excitation voltage. When the central heat source mass block is excited and forms a dynamic thermal field, the thermal field distribution around the central heat source mass block changes under the action of the input X-axis and Y-axis accelerations, causing a temperature difference between the thermistors on both sides that is related to the input acceleration, which is then converted into a corresponding electrical signal output.

[0013] As a further supplement to the above technical solution, metal electrodes, preferably Ag electrodes, are provided at both ends of the central heat source resistor and the thermistor. The Ag electrodes can be formed by magnetron sputtering.

[0014] The fabrication process of the all-PI laser-induced graphene switch-triggered flexible uniaxial accelerometer of this invention is as follows: 1. A rectangular structural layer substrate is formed on a flexible material PI by laser engraving, such as... Figure 7 As shown in (a).

[0015] 2. Graphene thermistors and central heat source resistors are formed on the PI structure layer by laser engraving, such as... Figure 7 As shown in (b).

[0016] 3. A honeycomb-shaped support structure and a central heat source mass block structure are formed by cutting with a laser engraving machine, such as... Figure 7 As shown in (c).

[0017] 4. Ag is sputtered using a magnetron sputtering instrument to form electrodes, which are then electrically connected to a graphene thermistor, such as... Figure 7 As shown in (d).

[0018] 5. Two large PI materials of the same size as the structural layer are cut using a laser engraving machine. A grooved top cover is formed on one of the materials using a dot matrix process. The other material serves as the base plate. The top cover, structural layer, and base plate are sequentially bonded to the structural layer to complete the encapsulation of the flexible accelerometer's sensing element. Figure 7 As shown in (e).

[0019] By adopting the above technical solution, the present invention has the following beneficial effects: (1) The top cover, structural layer and base plate are made of full PI flexible material, which is beneficial to improve the device’s adaptability to curved surfaces and flexible carriers, and reduce the mechanical and thermal mismatch between different materials, thus reducing temperature drift and time drift.

[0020] (2) A honeycomb heat source support structure is used to suspend the central heat source mass block, which is suitable for lightweight and stable sensitive structure support under flexible material conditions.

[0021] (3) Laser-induced graphene is used to form a central heat source resistor and a thermistor, eliminating the need for additional preparation of a platinum thermistor. The process is simple and the cost is low.

[0022] (4) Using slender graphene resistors as thermistors has the advantages of small heat capacity, low thermal inertia and fast response speed, and can improve the sensitivity to temperature changes.

[0023] (5) Integrated processing of devices can be completed through laser ablation, laser cutting and laser dot matrix processes, which facilitates digital manufacturing and mass production. Attached Figure Description

[0024] Figure 1 A schematic diagram of the disassembled structure of the all-PI laser-induced graphene switch-triggered flexible single-axis accelerometer provided in this embodiment of the invention.

[0025] Figure 2 A schematic diagram of the honeycomb support structure for a fully PI laser-induced graphene switch-triggered flexible single-axis accelerometer provided in this embodiment of the invention.

[0026] Figure 3 A schematic diagram of the graphene resistor structure of a fully PI laser-induced graphene switch-triggered flexible single-axis accelerometer provided in this embodiment of the invention.

[0027] Figure 4 A schematic diagram of the base plate structure of the all-PI laser-induced graphene switch-triggered flexible single-axis accelerometer provided in this embodiment of the invention.

[0028] Figure 5 A schematic diagram of the upper cover structure of the all-PI laser-induced graphene switch-triggered flexible single-axis accelerometer provided in this embodiment of the invention.

[0029] Figure 6 A schematic diagram of the extraction circuit for a fully PI laser-induced graphene switch-triggered flexible single-axis accelerometer provided in this embodiment of the invention.

[0030] Figure 7 The present invention provides a process flow diagram for the fabrication of a fully PI laser-induced graphene switch-triggered flexible single-axis accelerometer element.

[0031] Icons: 1-Flexible top cover, 2-Honeycomb support structure, 3-Central heat source mass block, 4-Graphene thermistor, 5-Graphene balancing resistor, 6-Electrode, 7-Flexible structural layer, 8-Flexible base plate. Detailed Implementation

[0032] The present invention will be further described below with reference to the accompanying drawings and specific embodiments. It should be understood that the following embodiments are only used to explain the present invention and are not intended to limit the scope of protection of the present invention.

[0033] like Figures 1 to 7 As shown, this invention provides a fully PI laser-induced graphene switch-triggered flexible single-axis accelerometer and its fabrication method, comprising a flexible upper cover 1, a flexible structural layer 7, and a flexible base plate 8. The flexible upper cover 1 and the flexible base plate 8 are respectively disposed on the upper and lower sides of the flexible structural layer, and together with the flexible structural layer, form an internal cavity of the device. A central heat source mass block 3 is disposed in the middle of the flexible structural layer 7, and the central heat source mass block 3 is connected to the surrounding fixed area through a honeycomb support structure 2. A laser-induced graphene heat source resistor is formed on the central heat source mass block 3, and laser-induced graphene thermistors 4 are symmetrically disposed on both sides of it. A graphene balancing resistor 5 is also disposed on the flexible structural layer, forming a bridge detection circuit with the graphene thermistors 4. During operation, a constant current is input to the central heat source mass block 3 for heating, forming a symmetrical temperature field around it. The graphene thermistors 4 on both sides detect the same temperature. Once an acceleration is applied along the plane of the structural layer, the central heat source mass block 3 shifts under the action of inertial force, breaking the heat flow symmetry. This causes a differential temperature change in the pair of thermistors paired along the sensitive axis. With the help of the excellent mechanical flexibility of the polyimide substrate and the high thermal sensitivity of the laser-ablated graphene resistors, this change is converted into a voltage signal proportional to the acceleration through a Wheatstone bridge. At the same time, the accelerometer still has a stable heat flow field detection capability in a bending state, realizing acceleration switch-triggered measurement in a flexible environment.

[0034] In this embodiment, the central heat source mass block 3 is suspended and connected using a honeycomb support structure 2. The honeycomb support structure 2 is made of a porous material with a small cross-sectional area, light weight, high flexibility, and easy movement under inertial forces. The honeycomb support structure 2 is composed of multiple regular hexagonal unit arrays. The hexagonal units are symmetrical and uniformly distributed, giving the honeycomb support structure 2 similar mechanical properties in all directions, achieving isotropy. Simultaneously, the two-dimensional honeycomb support structure 2 can evenly distribute external forces to each unit, avoiding localized stress concentration, and possesses high compressive strength and resistance to compressive deformation. Therefore, this structure has high stiffness and good stability.

[0035] The sensitive element of this invention uses a fully PI flexible material. The graphene thermistor 4 is generated in situ on the PI substrate through laser etching, eliminating the need for platinum metal sputtering and achieving near-zero cost fabrication. The all-PI homogeneous material eliminates internal stress mismatch, significantly suppressing temperature and time drift and ensuring long-term stability. The selection of a slender graphene resistor as the graphene thermistor 4 combines the advantages of fast response speed and high sensitivity, while avoiding the manufacturing challenges of complex-shaped resistors. Furthermore, the graphene balancing resistor 5 and the graphene thermistor 4 are integrated onto the same PI substrate through synchronous laser induction, improving the initial balance accuracy of the bridge circuit, avoiding signal drift caused by mechanical / thermal mismatch, simplifying the process, reducing costs, and adapting to mass production.

[0036] In this example, the fabrication method for the all-PI laser-induced graphene switch-triggered flexible single-axis accelerometer uses a laser engraving machine as the core process equipment. First, a rectangular structural layer substrate is formed on the flexible PI material using a laser engraving machine. Then, graphene thermistors 4 and graphene heat source resistors are formed on the flexible structural layer 7 substrate by laser engraving. Subsequently, the outline, honeycomb support structure 2, and related local structures of the flexible structural layer 7 are processed using laser cutting and dot matrix processing functions. Ag electrodes 6 are formed by sputtering using a magnetron sputtering instrument and electrically connected to the graphene thermistor 4. After the structural layer processing is completed, the flexible top cover 1 and flexible bottom plate 8, cut by the laser cutting machine, are respectively placed on the upper and lower sides of the flexible structural layer 7 and encapsulated, thereby forming the working cavity of the device. Because laser engraving equipment has the characteristics of digital pattern processing and good process consistency, it is beneficial to improve the repeatability of the preparation process of the graphene heat source resistor and the detection thermistor 4. Furthermore, the device of the present invention can be connected to microcomputer embedded systems such as microcontrollers, and the overall performance of the device can be improved through temperature compensation and nonlinear compensation.

[0037] The working principle of this application is as follows: When a linear acceleration is input along the x-axis (or y-axis), the central heat source mass moves in the same direction as the acceleration, causing an asymmetrical temperature field distribution generated by the hot airflow. The temperatures of two adjacent thermistors TX1 and TX2 in the same direction change oppositely; the thermistor biased towards the central heat source mass has a higher temperature than the thermistor parallel to it, resulting in a temperature difference between the two adjacent thermistors TX1 and TX2. TX1 and TX2 are used as two arms of a Wheatstone bridge, with the other two arms being balancing graphene resistors R. The area of ​​R is twice that of the graphene thermistors, and its resistance is greater than or equal to TX1 or TX2. The graphene resistors R are positioned outside the cavities of the thermistors TX1 and TX2, respectively, and are unaffected by the hot airflow. The bridge is as follows: Figure 6 As shown. The temperature difference caused by the acceleration of the input line, according to the thermal resistance effect, is converted into a change in the resistance of the bridge arms, thereby causing an unbalanced voltage in the bridge that is proportional to the input acceleration. V The linear acceleration along the x-axis can be calculated based on the output voltage, thus sensing the acceleration in the X-direction and using it to apply the excitation voltage. When the central heat source mass block is excited and forms a dynamic thermal field, the thermal field distribution around the central heat source mass block changes under the action of the input X-axis and Y-axis accelerations, causing a temperature difference between the thermistors on both sides that is related to the input acceleration, which is then converted into a corresponding electrical signal output.

[0038] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A fully PI laser-induced graphene switch-triggered flexible uniaxial accelerometer, characterized in that, The system comprises a flexible top cover, a flexible structural layer, and a flexible base plate. All three components are made of polyimide (PI) flexible material. A central heat source mass block is positioned at the center of the flexible structural layer, suspended by a honeycomb support structure. A central heat source resistor formed from laser-induced graphene is mounted on the central heat source mass block, and laser-induced graphene thermistors are symmetrically arranged on both sides of the mass block. The central heat source resistor generates a dynamic thermal field, and the thermistors detect changes in the thermal field around the central heat source mass block under X and Y accelerations, outputting an acceleration-related electrical signal. The top cover and flexible base plate are respectively positioned on the upper and lower sides of the structural layer and connected to it, forming a cavity to accommodate the central heat source mass block and the thermistors. Metal electrodes are provided at both ends of the central heat source resistor and the thermistors for connection to an external excitation circuit and a signal extraction circuit. The electrical signal is triggered for switching output through threshold comparison.

2. The all-PI laser-induced graphene switch-triggered flexible uniaxial accelerometer according to claim 1, characterized in that, The central heat source mass block is a hexagonal mass block.

3. The all-PI laser-induced graphene switch-triggered flexible uniaxial accelerometer according to claim 1, characterized in that, The honeycomb support structure is arranged around the central heat source mass block and is formed by connecting multiple hexagonal honeycomb units.

4. The all-PI laser-induced graphene switch-triggered flexible uniaxial accelerometer according to claim 1, characterized in that, The thermistor is a pair of strip-shaped graphene thermistors symmetrically arranged on both sides of the central heat source mass block.

5. The all-PI laser-induced graphene switch-triggered flexible uniaxial accelerometer according to claim 1, characterized in that, The central heat source resistor and the thermistor are formed in situ on the flexible structure layer by laser-induced graphene using the same laser ablation process.

6. The all-PI laser-induced graphene switch-triggered flexible uniaxial accelerometer according to claim 1, characterized in that, The flexible structure layer is also provided with a balancing resistor, which together with the thermistor constitutes a Wheatstone bridge detection circuit.

7. The all-PI laser-induced graphene switch-triggered flexible uniaxial accelerometer according to claim 1, characterized in that, The metal electrode is an Ag electrode.

8. A method for processing the all-PI laser-induced graphene switch-triggered flexible uniaxial accelerometer according to any one of claims 1-7, characterized in that, The method includes the following steps: S1. Provide PI flexible material and cut the PI flexible material to form a structural layer substrate; S2. Laser-induced graphene central heat source resistor and thermistor are formed in situ on the substrate of the structural layer by laser engraving machine ablation process; S3. Laser cutting is performed on the structural layer substrate to form a honeycomb support structure and a central heat source mass block structure; S4. Ag is sputtered using a magnetron sputtering instrument to form an electrode and then electrically connected to a graphene thermistor. S5. Prepare an upper cover and a bottom plate that fit with the substrate of the structural layer, and form a groove structure on the upper cover by laser dot matrix process; S6. The top cover, the structural layer base and the bottom plate are sequentially bonded and sealed to obtain the flexible single-axis accelerometer.

9. The fabrication method of the all-PI laser-induced graphene switch-triggered flexible uniaxial accelerometer according to claim 8, characterized in that, In step S2, the central heat source resistor and the thermistor are used to form laser-induced graphene in situ on the structural layer substrate through the same laser ablation process.

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