Hollow stems for higher micromechanical disk resonator quality factor

The use of hollow support stems to reduce energy loss to the substrate while supporting all-polysilicon UHF micromechanical disk resonators has enabled quality factors as high as 56,061 at 329 MHz and 93,231 at 178 MHz - values now in the same range as previous disk resonators employing multiple materials with more complex fabrication processes. With a substantially smaller cross-sectional area compared with the full stems used by predecessors, the hollow stem of this work effectively squeezes the energy conduit between the disk structure and the substrate, thereby suppressing energy loss and maximizing Q for devices operating in radial-contour and whispering gallery modes. Measurements confirm Q enhancements of 2.6× for contour modes at 154 MHz and 2.9× for wine glass modes around 112 MHz over values previously achieved by full stem all-polysilicon disk resonators with identical dimensions. The measured results not only demonstrate an effective Q-enhancement method with minimal increase in fabrication complexity, but also provide insights into anchor loss mechanisms that have been largely responsible for limiting the Q's attainable by all-polysilicon capacitively-transduced MEMS resonators.