Silicon piezoresistors for MEMS pressure sensor applications

Abstract

Silicon based micromachining technology enables the realization of high perfonnance microelectromechanical systems (MEMS) including a range of physical and environmental sensors. Pressure sensors are used for a wide range of monitoring and control applications, e.g. environmental, industrial, aircraft, automotive. Monitoring of vehicle tyre pressures offers benefits such as improved safety, fuel economy, and tyre life. Micromachined pressure sensors are used at present, but require further research to improve their performance in terms of size, power consumption and manufacturing cost. This thesis reviews pressure sensor technology and new developments in this area. A comparison of existing and new sensing mechanisms has been undertaken and identified as silicon piezoresistors. The focus of the research is motivated by the recently discovered enhanced piezoresistive effect in silicon nanowires where sensitivity can be increased by decreasing the dimension ofthe nanowire. This thesis investigates the piezoresistive effect in /i-type silicon nanowires, fabricated using a top down approach. It is found that the piezoresistive effect increases when the nanowire is reduced below 400 nm. Compared with micrometre sized piezoresistors, silicon nanowires have produced up to 56% enhancement. In addition, measurements indicate that the temperature coefficient of resistance (TCR) of a silicon nanowire is reduced by up to 40% compared to the bulk value. The improvement in these two parameters will be beneficial for the development of new MEMS pressure sensors with better sensitivity and temperature stability. Commercial software, COMSOL is employed to simulate the piezoresistance effect in />-type silicon for a range of doping concentrations. Simulation v results demonstrate a similar trend to experimental results and publication data and show that the piezoresistance effect decreases as the doping concentration increases as expected. The outcome of this research is useful to improve the performance of existing tyre pressure sensor with enhanced sensitivity and reduced temperature dependancy.

Date of Award	Jul 2014
Original language	English
Awarding Institution	Queen's University Belfast
Supervisor	Neil Mitchell (Supervisor) & David McNeill (Supervisor)