Fabrication of DETF sensors in SOI technology with submicron air gaps using a maskless line narrowing technique

We report about the fabrication of double-ended tuning fork (DETF) single-crystal silicon sensors starting from SOI substrates in which a novel line narrowing technique is adopted in order to shrink the electrostatic coupling gaps between the moving and fixed electrodes. By using conventional near-UV lithography, this solution provides the possibility to control gaps scaled down to 200 nm on an oxide hard mask realized on the structural silicon layer, yielding air gaps with minimum feature size between 400 and 600 nm after the subsequent silicon deep reactive ion etching (DRIE) step. With the proposed process, DETF structures with length varying between 100 and 500 mum have been realized, with a process yield around 85% on a 4-inch SOI substrate. DC testing of the realized prototypes, performed in a SEM-based setup, and vacuum AC testing are also presented, providing a first evaluation of the device pull-in voltage, resonance frequency and Q factor.