Active sensing in silicon-based MEMS resonators

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Electrical Engineering and Computer Science, 2015.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Cataloged from student-submitted PDF version of thesis.Includes bibliographical references (pages 139-148).Microelectromechanical resonators are advantageous over traditional LC tanks and o-chip quartz crystals due to their high quality factors, small size and low power consumption. FET-sensing has been demonstrated in resonant body transistors (RBTs) to reach an order of magnitude higher frequencies than possible with passive resonators due to the greater sensing eciency of FET sensing over traditional mechanisms such as capacitive or piezoelectric sensing. This thesis explores FET-sensing in Si-based MEMS resonators with dielectric and piezoelectric materials for design of fully unreleased CMOS-integrated resonators for multi-GHz frequency applications. Monolithic integration of Si-based MEMS resonators into CMOS is critical for commercial applications due to reduced size, weight and parasitics. A vast majority of CMOS-integrated resonators require a release step to freely suspend their vibrating structures, necessitating costly, complex encapsulation methods. This thesis proposes the development of fully unreleased resonators in CMOS using acoustic confinement structures, which may be realized without any post-processing or packaging. These di-electrically driven, FET-sensed resonators may be fabricated at the transistor-level of a standard CMOS process, and are demonstrated upto 11:1 GHz with quality factors (Q) up to 252 with footprints of less than 5[mu] x 7[mu]m with temperature coefficients of frequency (TCF) < 3 ppm/K. While electrostatic resonators have been primarily explored in this work due to the availability of such dielectric materials in a standard CMOS stack, piezoelectric materials remain popular in commercial MEMS resonators for their high electromechanical coupling factors. Recent years have seen a push towards integration of piezoelectric materials into standard CMOS for switching and memory applications. This work explores the performance improvements arising from the integration of CMOS-ready piezoelectric materials such as AlN into a resonant body transistor. This is shown to improve transduction eciency for low insertion losses at multi-GHz frequencies, for applications in communications to microprocessor clocking.by Radhika Marathe.Ph. D