A new technology based on a combination of Al-protection layers and HF-vapor etching to produce ultrathin single crystal silicon cantilevers is presented. 500 um long, 10 um wide and 0.5 um thick cantilevers have been fabricated with a high yield. A resonance frequency of 2 kHz, Q factor >100,000 and a force sensitivity of 6.0 x 10^17 N/Hz^1/2 have been obtained in vacuum at room temperature for cantilevers annealed at 800 degrees C.