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Piezoelectric MEMS Devices in CMOS-Compatible Platform
Integrating piezoelectrics with the standard complementary metal-oxide-semiconductor (CMOS) process presents new opportunities for monolithic microelectromechanical systems (MEMS) with scaled size, weight, and power (SWaP). Unlike traditional electrostatic transduction in CMOS platform, piezoelectric actuation allows higher sensitivity and electromechanical coupling, and as such, has been recently adopted by foundries like TSMC and Globalfoundries in their standard CMOS lines for commercial MEMS applications. In this research, a tunable ferroelectric capacitor (FeCAP)-based unreleased RF MEMS resonator is presented, integrated seamlessly in Texas Instruments’ 130nm ferroelectric random-access memory (RAM) process. To achieve high-quality factor (Q) of the resonator, acoustic waveguiding for vertical confinement within the CMOS stack is studied and optimized. The FeCAP resonator is demonstrated with fundamental resonance at 703 MHz and Q of 1012. This gives a frequency-quality factor product which is 1.6x higher than the most state-of-the-art Pb(Zr, Ti)O3 (PZT) resonators. Moreover, ferroelectric poling parameters are investigated to demonstrate bias-dependent pole/zero transitions accompanied by 180° phase shift in multiple mechanical modes of the device. The resonator’s Butterworth-Van Dyke (BVD) model is modified to capture this unique switching and frequency-hopping mechanism. The designs are monolithically integrated into solid-state CMOS technology, with no post-processing or release etch step which is typical of other MEMS devices. These novel switchable resonators may have promising applications in on-chip timing, ad-hoc radio front ends, and chip-scale sensors.
In order to best leverage these new CMOS-MEMS resonators for RF signal processing, synchronized clock arrays, and on-chip sensors requiring a network of such resonators working together, we require the ability to route and manipulate mechanical signals within the CMOS stack. At high frequencies of operation, it is particularly important to minimize propagation losses and control the dispersion of elastic waves. Therefore, this research also proposes the design of new acoustic metamaterials in order to localize, guide, and split elastic waves with low dispersion. These designs will be prototyped in an AlN piezoelectric platform for the proof of concept but can translate with small modifications to direct CMOS implementation.