3D integration of micro electromechanical systems (MEMS) and integrated circuits (ICs) represents a typical "More than Moore" scenario: More (hetero) functions are added at the same time as systems are miniaturised by smart system integration. However, 3D integration of MEMS can be even more challenging than 3D IC integration due to the additional mechanical aspects. Both 3D integration of planar silicon sensors and bulk micromachined MEMS devices has been evaluated. The choices of 3D stacking technologies are discussed and common key process steps are recognised to be deep reactive ion etching (DRIE) and wafer bonding. Through silicon vias (TSVs) based on doped silicon or polysilicon are found to be applicable for stacking of 200-300m thick devices with a moderate demand for conductivity. For MEMS devices with a low I/O count, gold stud bump bonding (Au SBB) was found as a well suited and cost-effective stack formation technique in combination with such TSVs. The use o f plated microbumps or solid liquid interdiffusion (SLID) intermetallic compounds are alternative stacking approaches for devices with higher I/O counts. 3D integration of MEMS is expected to be an enabling technology of high-volume production for More than Moore applications as e.g. miniaturised wireless sensor nodes.
