Ultra-high aspect ratio trenches in single crystal silicon with epitaxial gap tuning

A new method for the formation of narrow (<;100nm), smooth, ultra-high aspect ratio (>500:1) trenches in monocrystalline silicon is demonstrated using a combination of deep reactive ion etching (DRIE) and epitaxial deposition. Aspect ratios achieved were an order of magnitude higher than using DRIE only. This method was used to fabricate width-extensional mode resonators with narrow gaps within our wafer-scale epitaxial polysilicon encapsulation process. A representative 52 MHz width-extensional mode resonator with a motional impedance near 1 KΩ is carefully characterized.