Abstract
This paper introduces the first tunable ferroelectric capacitor (FeCAP)-based unreleased RF MEMS resonator, integrated seamlessly in Texas Instruments’ 130 nm Ferroelectric RAM (FeRAM) technology. The designs presented here are monolithically integrated in solid-state CMOS technology, with no post-processing or release step typical of other MEMS devices. An array of FeCAPs in this complementary metal-oxide-semiconductor (CMOS) technology’s back-end-of-line (BEOL) process were used to define the acoustic resonance cavity as well as the electromechanical transducers. To achieve high quality factor (Q) of the resonator, acoustic waveguiding for vertical confinement within the CMOS stack is studied and optimized. Additional design considerations are discussed to obtain lateral confinement and suppression of spurious modes. An FeCAP resonator is demonstrated with fundamental resonance at 703 MHz and Q of 1012. This gives a frequency-quality factor product f cdot Q = 7.11 times 10^{11} which is 1.6× higher than the most state-of-the-art Pb(Zr,Ti)O3 (PZT) resonators. Due to the ferroelectric characteristics of the FeCAPs, transduction of the resonator can be switched on and off by adjusting the electric polarization. In this case, the resonance can be turned off completely at ±0.3 V corresponding to the coercive voltage of the constituent FeCAP transducers. These novel switchable resonators may have promising applications in on-chip timing, ad-hoc radio front ends, and chip-scale sensors.
Highlights
Introduction Tunable highQ, small footprint resonators have great potential in both mature and emergent fields such as radio frequency (RF) components[1,2] and communication[3], timing[4,5], sensing[6,7,8], and imaging[9]
This paper reports on the first piezoelectric resonators designed in Texas Instruments (TI)’s Ferroelectric RAM (FeRAM) process
The operational voltages for the ferroelectric capacitor (FeCAP) are limited between −1.5 V and 1.5 V by dielectric leakage
Summary
Introduction Tunable highQ, small footprint resonators have great potential in both mature and emergent fields such as radio frequency (RF) components[1,2] and communication[3], timing[4,5], sensing[6,7,8], and imaging[9]. The authors have previously demonstrated high-Q, unreleased resonators with field effect transistor (FET) electromechanical sensing, referred to as Resonant Body Transistors (RBTs)[11] in standard CMOS technology at frequencies ranging from 3 GHz12 to 32 GHz13. While the f·Q products of these devices are record breaking, their return loss and bandwidth are restricted by the fundamental limits of electrostatic transduction, which provides modest driving force density. This is evident in the case of planar CMOS technology (e.g. 32 nm SOI), where the electromechanical transconductance of a 3 GHz resonator is on the order of 100 nS14. It is necessary to explore alternative IC integrated materials
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