Considering the isotropic release process for nanoelectromechanical (NEM) devices, defining the specific sacrificial layer of the inter-metal-dielectric (IMD), i.e., the active region only for NEM devices, is one of the most important issue for complementary-metal-oxide-semiconductor-NEM (CMOS-NEM) co-integrated circuits. In this paper, novel fabrication method to define the active region of NEM devices is proposed by forming the trenched mesa-shape pattern in the IMD and depositing aluminum oxide (Al₂O₃) protecting layer. By applying the proposed process, the void space for mechanical operation of NEM devices can be formed user-controllably without the damage and collapse of CMOS part located below the NEM part. The feasibility of the proposed process is verified by fabricating and measuring the proof-of-concept prototype consists of the aluminum (Al) interconnects, silicon dioxide (SiO₂) IMD and NEM memory switches.