Research On Flexible Pressure Sensor With Laminated Structure Based On PDMS-Nanomaterial

With the development of MEMS technology and the emergence of new materials,various structures and fucntions of pressure sensors has been emerged,which provides effective technical support for efficient,non-destructive and convenient pressure dection.Although the traditional silicon-based sensor has good performance,it can not be conformal with the measured object due to its rigidity and brittleness,which limits its application in wearable devices.However,it is difficult for the current flexible pressure sensor to maintain high sensitivity while ensuring a large pressure detection range.In view of the above problems and typical testing requirements of physical sign pressure monitoring,we have proposed the flexible pressure sensors based on PDMS-bilayer graphene,PDMS-graphene foam and PDMS-silicon nanomembrane respectively.By analyzing the sensitive characteristics of different nanomaterials,we made deep study in the selection and preparation of pressure sensitive materials,sensor structure design and optimization,sensor performance testing,analyzing and other aspects,which improved the sensitivity of the flexible pressure sensor and pressure sensing ability effectively and layed a certain foundation for its application in related fields.The main research content of this paper is as follows:1.Theoretical analysis of electron transport in nanomaterials.The related properties and preparation methods of nanomaterial graphene were briefly introduced,and the band structure of graphene was systematically analyzed.In addition,the principle of giant piezoresistive effect of silicon nanomembrane was introduced in detail.The mechanoelectric coupling model of silicon nanomembrane based on the surface depletion mechanism was established.The influence of surface depletion layer thickness on the conductivity of silicon nanomembrane under two different states of partial depletion and complete depletion state was systematically analyzed,and the relationship between the conductivity of silicon nanomembrane and the surface potential was deduced.Comsol Multiphysics software was used for numerical simulation to verify the model,and the mechanism of giant piezoresistive effect of silicon nanomembrane was further investigated clearly.2.The research on flexible pressure sensor of PDMS-bilayer graphene.The corresponding3D model was established by Comsol Multiphysics software and the stress distribution was simulated and analyzed after applying vertical pressure on the flexible substrate PDMS,and the optimal size of conical microstructures on the flexible substrate was determined.A prepraration method of micro-cone PDMS substrate was proposed,and the micro-cone PDMS substrate was fabricated by photolithography,deep reactive ion etching and turning mold.After releasing the bilayer graphene in deionized water transferred it to a flat flexible substrate PDMS,then covered the flexible substrate with mico-cone structure on it and coated with silver paste then encapsulated it,then the PDMS-bilayer graphene flexible pressure sensor with laminated structure was prepared.The performance of the laminated flexible pressure sensor was tested by relevant instruments.Results show that the operating pressure range of the device is 0-20k Pa,while maintaining a sensitivity of 0.122 kpa^-1 in the pressure range of 0-5 k Pa,and 0.077kpa^-1 in the pressure range of 5-20 k Pa respectively.The response time is only 70 ms,the repeatability and durability of the sensor are excellent,and it can realize the perception of weak pressure well.3.The research on flexible pressure sensor of PDMS-graphene foam.A PDMS-graphene foam flexible pressure sensor based on the micro-cone PDMS flexible substrate was developed.The influence of the PDMS substrate with or without micro-cone structure on the sensitivity of the device was compared and the characteristics of the sensor was characterized and tested.The experimental results show that the pressure range of the microcone PDMS-graphene foam flexible pressure sensor is 0-25 k Pa,and the sensitivity is improved to 0.177 k Pa^-1.After many times of bending,it still has relatively stable working performance,and it also has good flexibility and recoverability.4.The research on the flexible pressure sensor of PDMS-silicon nanomembrane.A preparation and transfer method of silicon nanomembrane structure was proposed.Different sizes of silicon nanomembrane structure were prepared through a series of processes such as photolithography,etching,heavy doping,sputtering,and erosion.The silicon nanomembrane with a thickness of only 340 nm and the metal wires were successfully transferred to the PDMS flexible substrate by means of controlling the viscosity/thickness of PDMS,and flexible pressure sensor with laminated structure was prepared by printing metal electrodes on boths sides of the silicon nanomembrane and encapsulation.The piezoresistive coefficient of the silicon nanomembrane and the performance of the sensor were tested by P66 piezoelectric nanopositioning platform,normal temperature probe platform,semiconductor analyzer and pressure testing machine.Results show that the piezoresistive coefficient of silicon nanomembrane is about 7.106×10^-9 Pa^-1,which is 9.89 times higher than that of bulk silicon.The operating range of the flexible pressure sensor is extended to 200 k Pa.At the same time,it has excellent performance such as fast response（response time 85 ms）and long-term stability.The PDMS flexible substrate enable it to better fit the human skin and sense human motion signals accurately such as wrist joints bending.The fabrication method of flexible pressure sensors based on micro-cone PDMS-bilayer graphene and micro-cone PDMS-graphene foam was proposed in this study,which improved the poor forming effect of top layer micro-structure and the sensitivity of the flexible pressure sensor made of graphene material effectively.The double transfer method of silicon nanomembrane based on the viscosity/thickness regulation of PDMS was proposed to solve the problem the silicon nanomembrane and metal were difficult to transfer at the same time.And a flexible pressure sensor based on PDMS-silicon nanomembrane was developed,and the piezoresistive coefficient of silicon nanomembrane prepared is 9.89 times higher than that of bulk silicon.The detection range of the flexible pressure sensor is extended to 200 k Pa at the same time.The flexible sensors developed in this study solve the problem that this kind of device is difficult to meet both high sensitivity and large pressure detection range at the same time,and provide a new idea for the application of silicon-based pressure sensors in the wearable field.These flexible pressure sensors have great application prospects in human health monitoring and human-computer interaction.