3-D Integration of MEMS and CMOS Using Electroless Plated Nickel Through-MEMS-Vias

Three-dimensional integration of microelectromechanical systems (MEMS) and CMOS is able to achieve hetero-integrated microsystems with high performance, small size, low cost, and multiple functions. This paper reports a 3-D integration method using wafer transfer technology and electroless Ni plating with a noncontact induction (ENPNI) technique. Wafer transfer technology based on adhesive bonding and wafer thinning allows stacked integration of MEMS on top of CMOS. To address the challenge in fabricating through-MEMS-vias (TMVs) with small diameters, ENPNI has been developed to fabricate Ni TMVs as both the mechanical supports for suspended MEMS and the electrical connects between MEMS and CMOS. High-density MEMS arrays and CMOS circuits are integrated to demonstrate the feasibility of the 3-D integration method, and thermal cycling and mechanical vibration are performed to evaluate the reliability. Experimental results show that the 3-D integration processes do not exert distinct influences on MEMS arrays and CMOS circuits, and the integrated systems have good yield, uniformity, and reliability. The capability of ENPNI in the fabrication of small TMVs enables 3-D integration of high-density MEMS arrays and CMOS circuits, such as micromirror arrays, infrared focal planes, acoustic sensor arrays, radiation sensor arrays, and so on.