Capacitive-piezo Transducers for Higher Q Contour-mode Aln Resonators at 1.2ghz

A “capacitive-piezo” transducer that combines the strengths of capacitive and piezoelectric mechanisms to achieve an impedance and Q simultaneously lower and higher, respectively, than otherwise attainable by either mechanism separately, has allowed demonstration of a 1.2-GHz contour-mode AlN ring resonator with a motional resistance of 889 Ω and Q=3,073 higher than so far measured for any other d31-transduced piezoelectric resonator at this frequency. Here, the key innovation is to separate the piezoelectric resonator from its metal electrodes by tiny gaps to eliminate metal material and metal-to-piezoelectric interface losses thought to limit thin-film piezoelectric resonator Q’s, while also maintaining high electric field strength to preserve a strong piezoelectric effect. In addition, this capacitive-piezo transducer concept does not require dc-bias voltages and allows for much thicker electrodes that then lower series resistance without mass loading the resonant structure. The latter is especially important as resonators and their supports continue to scale towards even higher frequencies. INTRODUCTION The ever-increasing appetite for wireless interconnectivity is beginning to drive new functions, like frequency gating spectrum analysis [1], that in turn drive a need for GHz resonators with simultaneous high Q (>30,000) and low impedance (<200 Ω). Unfortunately, no single on-chip resonator device can deliver such performance in this frequency range. Indeed, among popular resonator choices, thin-film piezoelectric (e.g., AlN) resonators post lower electrical impedances, but also lower mechanical Q’s (e.g., Rx=125 Ω and Q=2,100 [2]), than capacitive counterparts (e.g., Rx=12.8 kΩ and Q=48,048 [3]) at comparable (~60 MHz) frequencies. To achieve simultaneous high Q and low impedance, either the impedance of capacitive resonators must be lowered [4][5], or the Q’s of piezoelectric resonators must be raised. This work focuses on the latter and specifically introduces a new “capacitive-piezo” transducer, shown in Fig.1(b), that combines the strengths of capacitive and piezoelectric mechanisms to achieve an impedance and Q simultaneously lower and higher, respectively, than otherwise attainable by either mechanism separately. Using this new transducer, a 1.2-GHz contour-mode AlN ring resonator achieves a motional resistance of 889 Ω and a Q=3,073 higher than so far measured for any other d31-transduced piezoelectric resonator at this frequency. The key innovation here is to separate the piezoelectric resonator from its metal electrodes by tiny gaps to eliminate metal material and metal-to-piezoelectric interface losses thought to limit thin-film piezoelectric resonator Q’s, while also maintaining high electric field strength to preserve a strong piezoelectric effect. To understand the logic behind this approach, the next section starts off with some discussion of previous attempts to raise piezoelectric resonator Q’s. RAISING PIEZOELECTRIC RESONATOR Q Indeed, plenty of researchers have sought to raise the Q’s of thin-film piezoelectric resonators, with approaches that span from reducing electrode roughness [6], to optimizing the electrode material [7], to carefully balancing the AlN-to-electrode thickness ratio [8], to use of a Bragg reflector to prevent energy loss [9]. Unfortunately, none of the above methods raises the Q’s of on-chip piezoelectric resonators anywhere near the >30,000 values needed for RF channel-selection and frequency gating spectrum analyzers. Yet, polysilicon resonators easily achieve such Q values (but with higher than-desired impedances). To date, the measured Q’s of polysilicon resonators are on the order of 20 times larger than that of sputtered AlN resonators at similar frequencies. Interestingly, material loss theory [10][11][12] predicts that the (f·Q) product limit due to (dominant) phonon-phonon interactions in the AlN material itself is only four times lower than that of silicon. This suggests that the AlN material itself might not be the principal culprit among Q-limiting losses, but rather the metal electrodes or the electrode-to-resonator interface strain might be more responsible. In fact, experimental data shows that as the thickness of a piezoelectric resonator’s electrode increases, both the resonance frequency and Q of the resonator drop due to mass loading and electrode loss, respectively [13]. Electrode-derived energy loss perhaps also contributes to the lower Q’s measured in d31-transduced resonators, where the electrodes often cover locations with the maximum strain, versus the Q’s of d33-transduced thickness-mode resonators, where electrodes are placed very close to the nodes of the acoustic standing waves. Of course, despite their lower Q’s, d31transduced resonators are arguably more attractive than d33, since their frequencies are set by CAD-definable lateral dimensions, so are more suitable for on-chip integration of multiple frequencies. Whether a resonator uses d31 or d33, both share the common problem that Q gets worse as dimensions scale to achieve larger coupling and/or higher frequencies. In particular, while a piezoelectric structure can be scaled, its electrode thickness often cannot scale as aggressively, since doing so incurs excessive electrical loss derived from increased electrode and interconnect electrical resistance. If a designer attempts to compensate for this by using thinner, but wider, metal traces, then the beams supporting the resonator would need to be wider to accommodate the wider metal traces, and wider beams incur more energy loss through supports (1) Reverse Piezoelectric Effect (3) Induced Charges on Electrodes d1 t d3 DP vi Thicker Electrode Thicker Electrode (2) Piezoelectric Effect (b) Input Electrode Output Electrode