In this work, we present a novel unidirectional transducer design for frequency scaling aluminum nitride (AlN)-based radio frequency (RF) microsystems. The proposed thickness-field-excited single-phase unidirectional transducers (TFE-SPUDT) adopt 5/16 wavelength electrodes and, thus, enable efficient piezoelectric transduction with better frequency scalability. The design space of the TFE-SPUDT is theoretically explored and validated using the acoustic delay line (ADL) testbeds. The ADL testbeds with a large feature size of 3 µm show a center frequency of 1 GHz, a minimum insertion loss (IL) of 4.9 dB, and a fractional bandwidth (FBW) of 5.3%, significantly surpassing the IL and frequency scalability of the previously reported AlN transducers. The design approach can potentially contribute to various AlN-based RF microsystems for signal processing, physical sensing, optomechanical interaction, and quantum acoustic applications, and are readily extendable to other piezoelectric platforms. Index Terms-Acoustic delay lines (ADLs), aluminum nitride (AlN), lamb mode, piezoelectric transducers, radio frequency (RF) microsystems, unidirectional transducers.
I. INTRODUCTIONA LUMINUM nitride (AlN)-based radio frequency (RF) microsystems have been widely adopted in the past decade for various applications, including RF acoustic signal processing [1]-[6], physical sensing [7], [8], optomechanical interaction [9]-[14], and quantum acoustics [15]. Fundamentally, their ubiquity owns to the desirable piezoelectric, mechanical, and optical properties of AlN [16], [17], which in combination provides a cross-cutting platform that allows the selection and combination of technical advantages from the electrical, optical, and acoustic domains. Often, effective electromechanical transduction is paramount in these systems as it holds the key for coupling signal and power between domains and consequently harnessing underlying benefits from each domain (e.g., low loss and small size for acoustics).One challenge in attaining efficient transduction is to launch acoustic waves or phonons unidirectionally for devices involving waveguiding or acoustointeraction with optical or