Low-Impedance VHF and UHF Capacitive Silicon Bulk Acoustic Wave Resonators—Part I: Concept and Fabrication

This paper presents high-performance high-frequency single-crystal silicon (SCS) capacitive resonators. Long and thick bulk-micromachined resonating block structures, which are referred to as ldquosilicon bulk acoustic wave resonatorrdquo (SiBAR), are fabricated using the high-aspect-ratio poly and single crystalline siliconrdquo (HARPSS) fabrication process on silicon-on-insulator (SOI) substrates. Such resonators operate in their horizontal width extensional modes with quality factors in the range of 10000-100000. With their comparatively large electrode area and deep-submicrometer capacitive transduction gaps, such resonators have demonstrated comparatively low impedances for capacitive resonators that are well within the desired range for high-frequency electronic applications. Sub-kilo-Ohm total electrical resistances and extracted motional resistance as low as 200 are demonstrated for the fundamental width extensional modes of SiBARs in the very-high-frequency range. Resonant frequencies up to 1.55 GHz are demonstrated for the higher resonance modes of the capacitive SiBARs with comparatively low impedances. Part I of this paper presents the basic operation concepts and fabrication methodology for the HARPSS-on-SOI SiBARs. Extensive resonator measurement data, including temperature characteristics, are presented in Part II of this paper, and different frequency tuning approaches for temperature compensation of such resonators are discussed and investigated.