Tunable Silicon Bulk Acoustic Resonators with Multi-Face AlN Transduction Roozbeh Tabrizian and Farrokh Ayazi School of Electrical and Computer Engineering Georgia Institute of Technology Atlanta, USA roozbeh@gatech.edu; ayazi@gatech.edu Abstract—This paper presents tunable width-extensional mode bulk acoustic resonators that are piezoelectrically-actuated and sensed using thin layers of AlN on the sidewalls as well as the top surface. By using both longitudinal and transverse piezoelectric effects of conformally-sputtered AlN layers on sidewalls and top surface of a 20 m thick resonator, a low motional resistance of ~35 Ω was achieved for a 100 MHz silicon resonator operating in air. The motional resistance is improved by at least 10x compared to similar devices with capacitive transduction. Furthermore, it is shown that the resonance frequency of these piezoelectrically-transduced devices can be tuned by varying the electric signal power from 0 to 7 dBm. I. INTRODUCTION Lateral bulk acoustic wave (BAW) resonators implemented in single crystal silicon (SCS) are of great interest for signal processing applications. Since the resonance frequencies of such resonators are defined lithographically, devices with multiple frequencies can be implemented in the same batch and integrated with CMOS circuitry. Furthermore, superior acoustic properties of SCS such as high bulk acoustic wave velocity, small intrinsic dissipation in different frequency regimes [1, 2] and extended linear-elastic region alleviate the implementation of resonators with small form factor, improved power handling, and high quality factor (Q). Although advances in micromachining techniques (i.e., fabrication of small capacitive gaps, deposition of high quality piezoelectric thin films, etc.) have enabled the realization of high-frequency SCS BAW resonators [2], large motional resistance of these devices remains a major obstacle to utilize them in low insertion loss filters. Most transduction techniques that have been used to actuate and sense silicon BAW resonators [3-5] require a relatively large DC polarization voltage (V p ) to provide sufficient electromechanical coupling required for low motional resistances. Piezoelectric transduction, on the other hand, has the advantage of high electromechanical coupling without requiring V p . However, all demonstrations of lateral micromechanical resonators have so far used the transverse piezoelectric coefficient e 31 of a planar piezoelectric layer such as AlN [6-9]. While low motional resistances have been demonstrated using this technique for AlN-on-Si resonators [6], achieving low resistances (< 50 Ω) has been challenging due to limited transverse piezoelectric coupling, which is mainly a result of smaller e 31 compared to e 33 and small transduction area. To overcome this limitation, high-order modes are used in high-frequency lateral BAW resonators to increase the transduction area, which in turn leads to a larger die area [8]. Moreover, the efficiency of transverse piezoelectric transduction of silicon lateral BAW resonators is significantly degraded by the increase in proportional thickness of silicon in the resonator stack. However, thick silicon substrates are desirable for improvement of Q, power handling and linearity. Furthermore, the resonance frequency of BAW devices with transverse piezoelectric transduction cannot be tuned without sacrificing transduction area and considerably degrading the motional resistance. Using the technique presented here, in addition to the AlN layer on top surface of the resonator AlN layers on the vertical sidewalls of the resonator are simultaneously used to employ the larger longitudinal piezoelectric coefficient e 33 to actuate and sense bulk acoustic waves through the resonator sidewalls. In this configuration since Mo electrodes on the sidewalls connect equi-stress areas of AlN layers, charge cancellation is substantially reduced. Moreover, since transduction occurs on two sidewalls in addition to the top surface, it is scalable with resonator thickness which makes this method preferable for high-frequency resonators with small width (frequency-determining dimension). Because the sidewall AlN layers are mainly responsible for actuation and sensing, the top AlN layer can be dedicated to tuning purposes without a considerable reduction in effective transduction area. II. MULTI-FACE ALN TRANSDUCTION Figure 1 shows the cross-section of a silicon BAW resonator with multi-face AlN transduction. Since the top and bottom Mo electrodes are conformally deposited over the resonator surface, an input voltage signal results in electric fields E Top and E SW perpendicular to the top surface and sidewalls, respectively. E Top and E SW induce transverse and longitudinal mechanical stress in top and sidewall AlN layers, respectively (Fig. 1a), which results in excitation of the width-extensional bulk acoustic mode (Fig. 2). This resonance mode has an amplified strain in the AlN films on