Optimization Design Of Suspended Graphene Pressure Sensor

As the thinnest two-dimensional material,graphene has superior properties unmatched by traditional materials such as silicon.The ultra-high Young's modulus and carrier mobility of graphene with piezoresistive effect make it have broad application in the field of pressure sensing.Prototypes of piezoresistive suspended graphene pressure sensors have been fabricated and achieved extremely high sensitivity.However,there are still problems such as inaccurate guidance of theoretical model,low yield,and prominent temperature influence.Therefore,this paper is to optimize the suspended graphene pressure sensor,including improving the traditional piezoresistive effect model to guide the optimization design of the sensor,optimizing the process flow to improve the device yield,and studying the temperature characteristics of the device to guide the temperature compensation.The research contents of the thesis include:1.Improvement and analysis of the piezoresistive effect model of suspended graphene pressure sensor.The influence along with the mechanism of the cavity gas state,temperature,doping,initial stress of the membrane,etc.is investigated.Considering the non-negligible gap between traditional piezoresistive effect model and the actuality,the piezoresistive effect model is improved by taking the cavity gas state variation and temperature change into account.Then,the influence of cavity gas state change and temperature is quantitatively analyzed.It is found that when the cavity depth is small relative to the maximum central displacement of the film,the cavity gas state variation will reduce the sensitivity and accuracy of the sensor,while when the cavity depth is much larger than the maximum central displacement of the film,the influence is negligible.Temperature change greatly affects the accuracy of the sensor.2.Aiming at the incomplete structure and the low yield of the device and the temperature influence,the optimization design of the sensor structure and fabrication process are investigated.Firstly,the cavity is designed as a through hole under the guidance of the improved piezoresistive effect model.Then,the sensor structure is designed with temperature compensation.And then the fabrication process is optimized using the ultra-thin PMMA as supporting layer to increase the device yield.Finally,the through-hole etching and graphene transfer methods are mainly discussed,and the fabrication of the suspended graphene pressure sensor is completed.3.Test and analysis of suspended graphene pressure sensors.Firstly,the influence of the test voltage on the graphene resistance is studied to reasonably formulate the test scheme of the pressure response experiment and the temperature experiment.Then the pressure response experiment and the temperature experiment are carried out.The results of pressure response experiment show that the optimized sensor has higher sensitivity and the sensitivity of the through-hole(cavity depth of 240um)graphene sensor is higher than that of the graphene sensor with 150 nm deep cavity.The experimental results of pure graphene and suspended graphene pressure sensor show that both pure graphene resistance and graphene-PMMA film resistance are positively correlated with temperature while the graphene-PMMA film is more sensitive to temperature.Finally,the graphene pressure sensor with temperature compensation structure is tested.The relationship between temperature and sensor resistance is built and can be used for data compensation for temperature drift.