Fabrication of high aspect ratio self-aligned stepped polysilicon electrode for single-crystal silicon microstructure

This paper reports a self-aligned process for the fabrication of a high aspect ratio stepped polysilicon electrode with sub-micron capacitive gaps, which increases the coupling factor in capacitive driving and sensing. In contrast to the previous works, the polysilicon electrodes, which are formed by refilling the trenches and etching back, are anchored on the surfaces of steps in the trenches and etched by two-step deep reactive ion etching. The polysilicon electrodes are self-aligned and perfectly symmetric. Because of the `U'-shaped profile produced by the refilling process, the polysilicon electrode is less sensitive to a stress gradient in Low-pressure chemical vapor deposition (LPCVD) polysilicon layer. A thin layer of heavily doped polysilicon and a low-stress polysilicon layer, instead of uniform, heavily doped polysilicon, are used to form the polysilicon electrodes without sacrificing the electrical performance, which decreases the deposition time for LPCVD significantly due to the much faster deposition of normal low-stress polysilicon. To validate the study, a single-crystal silicon length extensional mode resonator with a 300-nm capacitive gap is fabricated based on this process. The frequency of the resonator is measured to be 25.5955 MHz, with a quality factor of 67,656 under 20 Pa.