Signal Processing using CMOS-MEMS Integrated Resonators

Abstract

A central part of the ubiquitous computing world of today is Wireless Sensor Networks (WSN), consisting of node-based components in a mesh that detects the environment around it. The sensing and radio part of the WSN node are typically off-chip components which are needed in order to realize the required performance. This thesis sets forth to investigate how to use MicroElectroMechanical Systems (MEMS) resonators as signal processing units, directly integrated in Complementary Metal-Oxide Semiconductor (CMOS) technology. By integrating MEMS resonators directly with CMOS, one can omit some of the typical off-chip devices and thus enable more compact and cost-efficient WSN nodes.<br>CMOS-MEMS resonator structures have been made by etching CMOS dies after being processed, thus defining structures from the metal layers offered in the CMOS process. Post-processing of CMOS dies was possible through a service known as Application Specific MEMS Process Service (ASIMPS) where Carnegie Mellon University (CMU) has etched and released the MEMS structures. This post-CMOS process was further developed by making MEMS out of two different 90 nm CMOS processes. Five different CMOS runs were performed, three at UiO and two at CMU in 0.35 μm, 0.25 μm and 90 nm CMOS processes from both Taiwan Semiconductor Manufacturing Company (TSMC) and ST Microelectronics (STM).<br>Different resonator topologies have been modeled, simulated and measured. A set of basic resonators were combined in order to make more advanced multiport MEMS resonators, enabling down-mixing of high-frequency signals to an intermediate-frequency. Composite resonators have been connected at selected nodal points in order to obtain higher order filtering characteristics. Higher-order MEMS filters were made in different ways and compared. Soft frequency tunable MEMS resonators and multi-mode features of composite MEMS resonators were investigated.<br>Composite MEMS resonators and CMOS amplifiers have been combined to convert the resonator current to a voltage, enabling voltage-to-voltage filters and mixer-filters. High gain, low-noise Trans-Impedance Amplifiers (TIAs) was made and different Trans-Impedance Amplifier (TIA) topologies were evaluated. The various combinations of MEMS and CMOS resulted in unique filtering capabilities with an increased Q-factor and low-noise performance.