INFLUENCE OF SILICON ON QUALITY FACTOR, MOTIONAL IMPEDANCE AND TUNING RANGE OF PZT-TRANSDUCED RESONATORS

This paper provides a quantitative comparison and explores the design space of PZT-only (Lead Zirconium Titanate) and PZT-onSilicon length-extensional mode resonators for incorporation into RF MEMS filters and oscillators. We experimentally measured the correlation of motional impedance (RX) and quality factor (Q) with the resonators’ silicon layer thickness (tSi). For identical lateral dimensions and PZT-layer thickness (tPZT), the PZT-on-Silicon resonator has higher resonant frequency (dominated by silicon), higher Q (5,100 vs. 140) and lower motional impedance (51 Ω vs. 205 Ω). However, PZT-only resonator demonstrated much wider frequency tuning range (5.1% vs. 0.2%). INTRODUCTION Numerous applications in wireless communications and sensor networks have motivated the development of on-chip, high-Q MEMS resonators and filters to realize portable radios that consume low power and operate at the global range of frequency standards. Dielectrically transduced thickness shear mode and contour-mode resonators have the capability to reach high frequency of operation while maintaining high Q and low motional impedance [1,2]. A band-pass filter can be constructed by electrically or mechanically coupling an array of these resonators. However, the inadequate effective coupling efficiency of the dielectrically transduced resonators limits the bandwidth of the filter, creating a demand for transducers with high electromechanical coupling coefficient. Aluminum Nitride (AlN) is a popular material for fabrication of thin film bulk acoustic wave (BAW) and contour-mode resonators and filters because it possesses a high acoustic velocity, high quality factor and post-CMOS integration capability [3,4]. However, intermediate frequency applications have driven the investigations for new piezoelectric/ferroelectric materials for resonators. At low frequencies, ferroelectrics like PZT are better suited as they avoid thick film requirements and reduce the area required for the filters. Furthermore, PZT exhibits larger electromechanical coupling coefficient than AlN, enabling the design of larger % bandwidth filters [5]. A resonator with dynamically tunable center frequency will not only overcome fabrication tolerances and thermal drift, but will also facilitate filter design that has the ability to discern bandwidths between 0.1 MHz to 5 MHz, enabling a handheld analog spectral processor with voltage dependent dynamic frequency trimming and tuning. PZT has been previously shown to have frequency tuning capabilities in [6,7]. However, PZT-only resonators are well known to have low quality factor (Q < 300) [5,8]. In order to overcome the low quality factor of PZT-only resonators, we developed a new fabrication technology to integrate PZT transduction with single-crystal silicon resonators. LENGTH-EXTENSIONAL MODE RESONATORS Previous results have shown that length-extensional mode vibrations can be excited by sandwiching a piezoelectric transducer (AlN, Zinc Oxide, PZT, etc.) between metal electrodes [4,9]. Figure 1: 3D schematic of PZT-on-silicon length-extensional mode resonator, and (inset) ANSYS mode shape. However due to poor crystallinity and repeatability of the piezoelectric thin-films, several research groups have chosen to use the piezoelectric material for actuation and sensing, while utilizing single-crystal silicon (SCS) as the resonating structure [10,11] (Figure 1). In this effort, PZT transduced resonators were fabricated with and without a 10 μm thick silicon device layer to explore the insertion loss and Q trade-offs between the two types of resonators. The frequency independent motional impedances of the fundamental mode of PZT transduced length-extensional mode two-port resonators are ) ...( 2 2 31 2 _ 1 d W E t t