Side-Supported Radial-Mode Thin-Film Piezoelectric-on-Silicon Disk Resonators

• Published in: IEEE transactions on ultrasonics, ferroelectrics, and frequency control Vol. 66; no. 4; pp. 727 - 736
• Main Authors: Shahraini, Sarah, Shahmohammadi, Mohsen, Fatemi, Hedy, Abdolvand, Reza
• Format: Journal Article
• Language: English
• Published: United States IEEE 01.04.2019; The Institute of Electrical and Electronics Engineers, Inc. (IEEE)
• Subjects: Acoustics; Alignment; Anchor loss; Anchors; Anisotropy; Computer simulation; Crystal structure; Crystallinity; Diamonds; Disks; Energy loss; Frequencies; Harmonic analysis; Isotropic material; Material properties; perfectly matched layer; Perfectly matched layers; Piezoelectricity; Q factors; Q-factor; quality factor; radial mode; Resonators; Silicon; silicon disk resonator; Single crystals; Substrates; Tethers; Thin films
• ISSN: 0885-3010; 1525-8955
• DOI: 10.1109/TUFFC.2019.2893121

In this paper, anisotropy of single-crystalline silicon (SCS) is exploited to enable side-supported radial-mode thin-film piezoelectric-on-substrate (TPoS) disk resonators. In contrast to the case for isotropic material, it is demonstrated that the displacement of the disk periphery is not uniform for the radial-mode resonance in SCS disks. Specifically, for high-order harmonics, nodal points are formed on the edges, creating an opportunity for placing suspension tethers and enabling side-supported silicon disk resonators at the very high-frequency band with negligible anchor loss. In order to thoroughly study the effect of material properties and the tether location, anchor loss is simulated using a 3-D perfectly matched layer in COMSOL. Through modeling, it is shown that eighth-harmonic side-supported SCS disk resonators could potentially have orders of magnitude lower anchor loss in comparison to their nanocrystalline diamond (NCD) disk resonator counterparts given the tethers are aligned to the [100] crystalline plane of silicon. It is then experimentally demonstrated that in TPoS disk, resonators fabricated on an 8-<inline-formula> <tex-math notation="LaTeX">\mu \text{m} </tex-math></inline-formula> silicon-on-insulator (SOI) wafer, unloaded quality factor improves from ~450 for the second-harmonic mode at 43 MHz to ~11500 for the eighth-harmonic mode at 196 MHz if tethers are aligned to [100] plane. The same trend is not observed for NCD disk resonators and SCS disk resonators with tethers aligned to [110] plane. Finally, the temperature coefficient of frequency is simulated and measured for the radial-mode disk resonators fabricated on the 8-<inline-formula> <tex-math notation="LaTeX">\mu \text{m} </tex-math></inline-formula>-thick degenerately n-type doped SCS, and the TFC data are utilized to guarantee proper identification of the harmonic radial-mode resonance peaks among others.