A semiconductor fabrication is described, wherein a MOS device and a MEMS device is fabricated simultaneously in the BEOL process. A silicon layer is deposited and etched to form a silicon film for a MOS device and a lower silicon sacrificial film for a MEMS device. A conductive layer is deposited atop the silicon layer and etched to form a metal gate and a first upper electrode. A dielectric layer is deposited atop the conductive layer and vias are formed in the dielectric layer. Another conductive layer is deposited atop the dielectric layer and etched to form a second upper electrode and three metal electrodes for the MOS device. Another silicon layer is deposited atop the other conductive layer and etched to form an upper silicon sacrificial film for the MEMS device. The upper and lower silicon sacrificial films are then removed via venting holes.