Theoretical Model And Packaging Research Of Thermoelectric Microwave Power Sensors

Currently,MEMS microwave power sensors have been widely used in fields such as radar,satellite communication,and microwave medicine.Among them,thermoelectric MEMS microwave power sensors,as the mainstream microwave measurement instrument,have advantages of good linearity,high sensitivity,and low power consumption.However,the heat generated by the end load resistance of the thermoelectric MEMS microwave power sensor not only transfers to the thermopile but also dissipates to the surrounding environment,which has an adverse effect on the output characteristics of the sensor.In order to solve this problem,the structure of the thermoelectric MEMS microwave power sensor is optimized in this work,a three-dimensional equivalent circuit model is established to study the temperature distribution and time constant of the sensor.A chip on board(COB)packaging technology is proposed to package the thermoelectric MEMS microwave power sensor.It provides theoretical and applied technical support for the design of microwave power sensor.This main content of the article includes the following parts:(1)Substrate optimization of thermoelectric MEMS microwave power sensor: The heat generated by the load resistor is dissipated to the substrate,which affects the output characteristics of the sensor.ANSYS simulation is used to study the effect of substrate thickness and the position and size of the substrate membrane on the temperature,based on which the substrate structure of the thermoelectric MEMS microwave power sensor is optimized.Simulation results show that after optimizing the substrate structure,the temperature of the sensor increases by 26.93 K.The substrate membrane structure has a significant effect on the temperature of the sensor,and a substrate membrane of 10 μm increases the temperature of the sensor by approximately 20 K.The substrate membrane should be placed at the boundary of the minimum temperature of the sensor.(2)Establishing 3-D equivalent circuit model of thermoelectric MEMS microwave power sensor:To study the temperature distribution and time constant of the sensor,a three-dimensional equivalent circuit model of the sensor is established based on the equivalent relationship of thermal-electric parameters.By establishing the model,the 3-D temperature distribution centered on the load resistor and the time constant of different thermocouple lengths are obtained.The sensitivity and time constant of the thermoelectric MEMS microwave power sensor are measured after fabrication to verify the effectiveness of the model.(3)Studying output resistance of thermoelectric MEMS microwave power sensor: Firstly,the factors affecting the resistance of the two arms of the thermocouple are analyzed,and then the changes in the output resistance of the sensor are measured at different microwave powers and frequencies.The experimental results show that the output resistance of the thermopile increases linearly with the input microwave power but decreases with the microwave frequency.(4)Packaging thermoelectric MEMS microwave power sensor: The sensor is packaged using COB technology.Firstly,a model is established to verify the feasibility of the packaging scheme,and then the bare die is packaged using COB technology.Finally,the packaged chip is tested,and the results show that the packaged sensor has a return loss of less than-10.50 d B in the X-band.The microwave characteristics deteriorate due to the electromagnetic coupling effect of the wire bonding and the transition effect from microstrip line to coplanar waveguide without vias.The sensitivity before and after packaging are 0.16 m V/m W@10GHz and 0.18 m V/m W@10GHz,respectively.The main reason for the higher sensitivity after packaging is that the current generated by the sensor before and after packaging is the same,but the impedance and output voltage become larger after packaging.