Noise Floor of MEMS-Based Piezoresistive Microphones

Professor Toshikazu Nishida,
Interdisciplinary Microsystems Group,
Department of Electrical and Computer Engineering,
University of Florida

ABSTRACT: There has been growing interest in microelectromechanical systems (MEMS) microphones with projected growth in market size from $140 million/year to nearly $1 billion/year in 2011. For audio applications, the primary transduction mechanism is capacitive as illustrated by the Knowles SiSonicTM microphone due to its low noise floor. However, piezoresistive transduction has been employed for aeroacoustic applications due to its robustness to humidity and pressure changes. Piezoresistive MEMS is fully CMOS compatible for fabrication of the transducer while capacitive MEMS requires specialized techniques to fabricate the narrow gap. This talk will discuss the issue of noise floor (minimum detectable signal (MDS)) in MEMS-based piezoresistive microphones and will first review thermal and 1/f noise mechanisms. Factors affecting the contribution of 1/f noise on the MDS will be discussed. The design space of MDS will be explored, and current understanding of process effects on 1/f noise in silicon piezoresistors will be reviewed. Technical issues include the relative contributions of bulk defects in the piezoresistor and near-interfacial defects at the passivation layer/silicon interface and the physical origin of the defects. Strategies for reduction of the noise floor of piezoresistive MEMS transducers will be discussed.
Toshikazu (Toshi) Nishida is currently an associate professor in the Department of Electrical and Computer Engineering (ECE) and an Affiliate Associate Professor in the Department of Mechanical and Aerospace Engineering (MAE) at the University of Florida. He is a co-founder of the Interdisciplinary Microsystems Group, currently a six-faculty (three ECE and three MAE faculty), forty-seven graduate student multidisciplinary group http://www.img.ufl.edu). His research interests include solid-state physical sensors and actuators, transducer noise, strained semiconductor devices, and reliability physics of semiconductor devices. He and his students are currently investigating strain effects in piezoresistive microelectromechanical systems (MEMS) transducers and advanced CMOS devices, noise mechanisms in piezoresistive MEMS transducers, MEMS and mesoscale piezoelectric transducers for vibrational energy reclamation, MEMS capacitive microphones, and biomedical applications of MEMS. He received his Ph.D. (1988) and M.S. degrees in Electrical and Computer engineering and B.S. degree in Engineering Physics at the University of Illinois at Urbana-Champaign. With colleagues and students, he has published over 100 refereed journal and conference papers and received three best paper awards. He also received the 2003 College of Engineering Teacher of the Year award. He holds five U.S. patents. He is a distinguished lecturer for the IEEE Electron Devices Society.