JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 21, NO. 2, APRIL 2012 379 Waveguide-Based Phononic Crystal Micro/Nanomechanical High-Q Resonators Saeed Mohammadi, Member, IEEE, and Ali Adibi, Senior Member, IEEE Abstract—In this paper, we report the design, analysis, fabri- cation, and characterization of a very high frequency phononic crystal (PnC) micro/nanomechanical resonator architecture based on silicon PnC slab waveguides. The PnC structure completely surrounds the resonant area, and the resonator is excited by a thin aluminum nitride-based piezoelectric transducer stack di- rectly fabricated on top of the resonator. This architecture highly suppresses the support loss of the resonator to the surroundings while providing mechanical support and electrical signal delivery to the resonator. Qs as high as 13 500 in air at a frequency of ∼134 MHz with a motional resistance of ∼600 Ω and 35-dB spurious-free range of ∼20 MHz are obtained. Comparing the Q of this resonator with the previously reported lateral bulk acoustic wave resonators with a similar stack of layers confirms the support loss suppression in this architecture. [2011-0160] Index Terms—Electromechanical system, phononic band gap, phononic crystal (PnC), resonator, waveguide. I. I NTRODUCTION P HONONIC crystals (PnCs) [1], [2] are inhomogeneous materials with periodic variations in their elastic (or me- chanical) properties. The dispersion characteristics of the PnCs can be engineered to achieve functionalities not obtainable by the conventional bulk materials. One of the intriguing character- istics of PnC structures is the possibility of achieving complete phononic band gaps (CPnBGs), i.e., frequency ranges in which the propagation of elastic waves inside the PnC structure is prohibited. CPnBGs can be the basis of realizing a variety of functionalities including, but not limited to, guiding, trapping, filtering, multiplexing, and demultiplexing of elastic (acoustic) waves. Such functionalities can be obtained by modifying portions of the PnC structure to form intentionally introduced defects. Recently, PnC slab (plate) structures, which have two- dimensional (2-D) periodicity and a finite thickness (of the order of the wavelength) in the third dimension [3], with large CPnBGs have been designed and implemented in platforms compatible with micro/nanomechanical (MM) systems and complimentary-metal-oxide-semiconductor (CMOS) technolo- gies [4], [5]. In PnC slab structures, elastic vibrations can be Manuscript received May 23, 2011; revised October 12, 2011; accepted October 14, 2011. Date of publication January 16, 2012; date of current version April 4, 2012. This work was supported in part by the National Science Foundation under Contract Number ECCS-0901800 (A. Weisshaar). Subject Editor G. K. Fedder. The authors are with the School of Electrical and Computer Engineer- ing, Georgia Institute of Technology, Atlanta, GA 30332 USA (e-mail: saeedm@gatech.edu; adibi@ece.gatech.edu). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JMEMS.2011.2174426 manipulated in the two dimensions of the plane of periodicity by the PnC structure while being confined within the finite thickness of the slab in the third dimension due to acoustic mismatch. The air (or vacuum) on top and bottom of a PnC slab (or membrane) decouples the elastic energy of the vibrations in the PnC slab from leaking into the substrate. Thus, the loss of the elastic waves in the PnC slabs is considerably lower than that of the surface acoustic wave-based 2-D PnC structures, where coupling to the substrate can take place [6], [7]. The possibility of realizing fundamental micromechanical (MM) devices, i.e., waveguides [8], [9] and high quality factor (or high-Q) resonators [10], which are the bases of integrated mechanical signal processing systems, have recently been demonstrated. Because of their unique properties, such PnC- based devices and systems may surpass their conventional MM counterparts for a variety of applications including wireless communications and sensing. Among several material systems proposed for such PnC structures, silicon (Si)-based systems have many advantages due to their low cost, the availability of CMOS fabrication tools that allow accurate and economical fabrication of Si structures, proper elastic properties of Si that are required for low-loss applications, and the possibility of integration of the acoustic devices with electronic and photonic functionalities [11]. De- veloping high-Q Si-based MM resonators for such high fre- quency applications are, therefore, of great interest for realizing compact and efficient devices with desired characteristics. To obtain high-Q resonators in Si-based systems, mechanical energy must be stored in a resonating structure with the lowest possible loss. To minimize the loss of elastic energy to the surroundings, in-plane propagating vibrations in solid slabs suspended in air or vacuum are utilized. Utilization of in-plane propagating vibrations allows for fabrication of resonators with different frequencies of resonance on the same substrate. Two mainstream methods of excitation of such resonators are the capacitive [12] and piezoelectric [13], [14]-based excitations. In such suspended high-Q resonators, supporting structures are required to hold the structure and/or for providing an electrical path to interrogate the resonator. Unfortunately, such supports are sources of loss (and therefore, reduction of the Q) in these resonators as mechanical energy can leak through them to the substrate. Since PnCs with CPnBGs can provide mechanical support to the resonating mass and can simultaneously prevent the propagation of mechanical energy, they can be excellent can- didates to suppress such support losses and hence improve the Q of the resonator. Therefore, there has been recent impressive publications on the subject of suppressing support loss of the 1057-7157/$31.00 © 2012 IEEE