A Unidirectional Transducer Design for Scaling GHz AlN-Based RF Microsystems

Abstract

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

Enabling Higher Order Lamb Wave Acoustic Devices With Complementarily Oriented Piezoelectric Thin Films

“…ADLs [69], [79], [80], and also higher-order Lamb wave ADLs in single-layer structures [47], [51]. The feature sizes and the film thickness are also listed.…”

Acoustic Loss in Thin-Film Lithium Niobate: An Experimental Study

“…Finally, the loss extraction framework based on unidirectional ADLs is readily extendable to other thin-film structures, such as the emerging high-quality ScAlN/AlN piezoelectric thin films [59]- [64], by using the reported AlN unidirectional ADL designs [65]. Thus, this reported acoustic loss analysis approach could facilitate the future development of thin-film low-loss wideband microsystems in various acoustic platforms.…”

Microwave Acoustic Devices: Recent Advances and Outlook

“…Recently, extensive characterizations on acoustic wave propagation loss in thin-film LN have shown a material Q limit of 6000, while devices only display Q ranging from 300 to 1000. Such studies leverage a new group of devices, named acoustic delay lines (ADLs) [51], [80]- [82]. ADLs are two-port devices with a pair of piezoelectric transducers on the opposite end of the acoustic waveguide.…”

“…One can directly extract acoustic propagation loss [86] with the identical transducer design but different waveguide lengths [87]. Low-loss and wideband ADLs at GHz have been built using S0 in AlN thin films [82], S0 [87], SH0 [88], A1 [89], and higher-order Lamb waves [90] in LN thin-films, and SH-SAW in solidly mounted LN thin-films [91], [92]. The propagation loss reported for GHz acoustic waves in LN is around 0.005 dB/λ and 0.03 dB/λ [90], and S0 shows the lowest loss.…”