Design Analysis and Sensitivity Enhancement of Piezoresistive Micro Pressure Sensors

Abstract

Piezoresistive micro pressure sensors are widely used for pressure measurement. There are a few design issues need to be addressed like temperature sensitivity, placement of piezoresistive element on the membrane, design of element size, etc. For better sensitivity the length of piezoresistor needs to be increased whereas the thickness needs to be reduced. For a square membrane, the maximum stress value occurs at the center of the edge and decreases rapidly towards the center of the membrane. Increase in length of the piezoresistor beyond a limit will develop huge design error, since the localized stress variations are not considered in the design. The solution could be breaking down the resistance into smaller pieces and connecting them in series and positioning them in the maximum stress region. This paper discusses about the design of such a piezoresistive micro pressure sensor with optimized position placement and element size. Initially, a square diaphragm piezoresistive pressure using classical governing equations is designed. Since, three of the physics such as structural mechanics, piezoresistive physics and electrical physics are involved in the design; Comsol Multiphysics 4.3 is used to make a comparative study of the model with analytical design. The stress variation over the diaphragm is analyzed and the length of the piezoresistive element is designed by restricting the element in the maximum stress region. A Matlab code is written to find the optimum element size, and number of elements to be connected in series. The material properties, dimensions of the membrane, initial resistance and sheet resistance of the piezoresistor are considered as variables and obtained as user inputs