A wafer-level sealed silicon cavity microacoustic platform for radio frequency integration.

Abstract

This study presents a wafer-level sealed silicon cavity (SSC) microacoustic integration platform to address the limitations in the cavity Silicon-on-Insulator (C-SOI) wafers for the 5G/6G wireless communication system. The proposed SSC platform features an extremely smooth suspended membrane with adjustable thickness, flexible cavity shapes with high density, self-formed acoustic wave confinement steps, stable temperature coefficient of frequency (TCF), and highly integrated compatibility with complementary metal-oxide semiconductor (CMOS). A surface smoothing method based on wet oxidation for SSC wafers is presented, which achieves a root mean square (RMS) roughness on the cavity surface of 1.5 nm for the first time. Based on the presented SSC platform, an Al_0.75Sc_0.25N sealed cavity bulk acoustic wave resonator (S-BAR) is designed, fabricated, and characterized. The experimental results show that the asymmetric second-order (A2) Lamb mode of S-BAR is enhanced for higher frequency with a maximum piezoelectric coupling coefficient ( $k_t^2$ ) of 9.53%, a maximum quality factor (Q) of 439, and a TCF of -11.44 ppm/K. Different designs' piezoelectric coupling coefficient distribution is consistent with the theoretical prediction. The proposed smoothing process increases the S-BARs' quality factor by ~400%. The frequency shift caused by the temperature (absolute value of TCF) is reduced by 62% compared with the traditional Al_0.75Sc_0.25N thin film bulk acoustic wave resonator (without temperature compensation). The enhanced performances demonstrated the potential of SSC in the next-generation highly integrated RF communication systems.