The authors present a novel approach to fabricate nanomechanical devices integrated into complementary metal-oxide-semiconductor (CMOS) circuits. It is based on focused ion beam patterning using two different processes: (i) ion-beam-induced deposition of tethraethoxysilane and (ii) direct exposure of silicon or polysilicon surfaces. In both cases, the irradiated areas sustain a reactive-ion etching process, acting as robust masks for defining nanomechanical devices with submicron resolution. These processes are compared, in terms of throughput, with direct milling of silicon and with patterning of thin aluminum layers. Compatibility with prefabricated CMOS circuits is studied and they found that the process is entirely compatible if the proper exposure conditions are used.