Perforated diaphragms employed piezoresistive MEMS pressure sensor for sensitivity enhancement in gas flow measurement

Abstract

This paper presents the details of a study on the measurement of oxygen flow by differential pressure method in a clinical ventilator system. The simulation results obtained from the COMSOL Multiphysics MEMS design tool show that the meso-channel with a diameter of 1000μm and length of 20mm can cause measurable pressure drop between the upstream and downstreams without altering the flow and therefore can be used as a flow resistor. Two piezoresistive MEMS pressure sensors are proposed to be installed at the upstream and downstream to measure the differential pressure and thus the gas flow rate. Further investigations on thin film silicon diaphragms with embedded piezoresistors for sensing the upstream and downstream pressures show that it is essential to employ thin diaphragms for pressure sensing in this application to achieve higher sensitivity with reasonably good linearity. However very thin diaphragms results in more non-linearity and are difficult to realize. Hence the authors have undertaken a study on perforated thick diaphragms for pressure sensing in piezoresistive MEMS pressure sensors for such applications. The IntelliSuite MEMS design tool has been used to create and analyze the performance of perforated diaphragm employed piezoresistive pressure sensors on 3μm, 5μm and 7μm thick diaphragms each with different side lengths of 500μm, 700μm and 900μm. The results show that it is possible to achieve more than 93% improvement in deflection sensitivity, more than 136% improvement in stress generation and 83% improvement in voltage sensitivity with 40 % perforated area irrespective of the thickness of the diaphragm. Empirical results on perforated diaphragms have been reported to be matching with COMSOL Multiphysics simulation results. Therefore the authors have simulated the perforated diaphragms studied in this work using COMSOL Multiphysics and compared with the IntelliSuite simulation results. The comparison confirms the validity of the results. A modified analytical model developed in this study for perforated diaphragm load–deflection performance shows that the simulation obtained for various pressure sensors employing perforated diaphragms in this study are accurate and valid. This leads to the conclusion that the perforation realized on thicker diaphragms are suitable alternatives with satisfactory performance to very thin non-perforated diaphragms. The flow using the piezoresistive pressure sensors employing perforated diaphragms for differential pressure measurement are found to be giving larger flow sensitivity than the 1differential flow sensors already reported in the literature. This work therefore demonstrates that it is possible to design micro-gas flow measurement system by differential pressure method using micro-pressure sensors with perforated diaphragms integrated with meso-channel.