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Design and Implementation of Piezo Resistive MEMS Pressure Sensor for Spiro Meter Application

D. Mary Magdaline Zeena

ABSTRACTSpirometer is an essential component in the medical evaluation of patients suffering due to shortness of breath. It takes measurement of the quantity of air inhaled and exhaled by the lungs during a certain period of time to determine the pulmonary capacity. Improved spirometers are used for estimating the severity of the airflow obstruction in the patients suffering with asthma and the impact of Chronic Obstructive Pulmonary Disease (COPD). Earlier days, volume displacing spirometers and flow sensing spirometers were used. However, volume displacing spirometers are large and bulky and hence they were less portable. The flow sensing spirometers are portable, smaller and measure flow. Undesirable characteristics of flow sensing is frequent and careful calibration checks needed for some models. Further moisture and secretions may cause problems in these devices. Nowadays low cost, portable spirometer built with pressure sensor is used for detecting airflow and pressure. In this present work, the design and analysis of MEMS pressure sensor employed spirometer has been carried out. The MEMS sensors ensure high sensing accuracy and good stability which are the essential requirements for spirometers used in advanced medical applications. This is achieved with polysilicon piezoresistors in the present work. As a part of the spirometer design the effect of diaphragm shape in the MEMS pressure sensor that is to be used in the spirometer is presented first. The circular and square diaphragms have been studied and a comparison study on the results paves the way for selecting the shape of the diaphragm to achieve very sensitive pressure sensors. Subsequently the results of the simulation studies conducted on various diaphragm sizes to design the geometries of the pressure sensor is presented. This design involves arriving at the best optimized sensitivity and linearity. IntelliSuite MEMS design tools that uses FEM analysis have been used in all the simulation studies. Finally the performance of these MEMS pressure sensors is presented.