Micromachined force detectors are extremely sensitive instruments capable of measuring forces as small as 10 −15 N. We describe one such instrument that combines a novel micromachined torsional oscillator with an interferometric position-sensing mechanism that allows fine control of vertical scans. The design, fabrication, and operation of MEMS-based force sensors are described. The sensitivity and unique features of micromachined torsional oscillators allow us to undertake experiments that set new constraints on corrections to Newtonian gravitational forces at separations below 1 mm. Moreover, the measurements provide the most precise characterization of quantum vacuum fluctuation forces to date. © 2005 Lucent Technologies Inc.