Micro-to-Nanoacoustic Scaln Lamb Wave Resonators: Frequency Scaling Towards The Mm-Wave Spectrum

In this work, we demonstrate 30% Scandium-doped Aluminum Nitride (ScAlN) nanoacoustic Cross-sectional Lamé Mode Resonators (CLMRs) up to 20 GHz of operation (K-band). The devices are fabricated on 200 mm wafers with a simple 3-mask micro-machining process and feature interdigitated electrodes’ finger widths ranging from 75 to 144 nm. Among them, we showcase a groundbreaking CLMR operating at 18.17 GHz with a 3dB quality factor (Q3dB) of 209 and a single-mode electromechanical coupling $\left({k_t^2}\right)$ of 6.12%, or 7.65% after de-embedding. This results in outstanding ${Q_{3dB}}\cdot k_t^2$ Figures of Merit (FoM) of 12.8 and 16, respectively. Moreover, the motional quality factor (Qm) of 226 yields a ${Q_m}\cdot k_t^2$ of 17.3, when the de-embedded $k_t^2$ is used. Such value, to the best of our knowledge, is the highest ever reported for a mechanical resonator operating in the K- band or above. Furthermore, the present metrics are obtained in an extremely compact form factor, with a device’s active area < 0.0002 mm². Finally, we draw conclusions about a further scaling of the CLMR technology to the mm-wave spectrum by providing finite element modeling and circuit simulations of AlN and lower Sc-doping level nanoacoustic resonators and filters, respectively. The presented scaling potentialities cast light on the present technology. In fact, the combination of CLMRs’ intrinsic lithographic tunability of the resonance frequency, high performance in the super high-frequency range, and simplified and post-CMOS compatible micro-machining process, can enable the mass production of low-loss and compact filtering devices to be used in handsets’ radiofrequency front-ends for the next-generation 5G cellular radios operating above 24 GHz.

INTRODUCTION

The last years have seen mass adoption of the fifth-generation mobile network (5G) in handheld devices, and the number of users is foreseen to keep increasing in the following years [1]. This new technological paradigm promised outstanding improvements in Key Parameter Indicators (KPIs) such as data-rate, spectral efficiency, and latency [2] compared to previous generations. However, despite an average increase in 5G download speeds for different United States carriers [3], the full 5G potential has not yet been exploited. In fact, mostly 5G sub-6 GHz (FR-1) bands were covered, leaving the mm-waves (f > 24 GHz, FR-2) almost unexplored in the framework of handsets due to a lack of high-performance, cost-effective, and compact hardware solutions.