Fabrication of High-Density Micro-Bump Arrays for 3D Integration of MEMS and CMOS

Cu-Sn transient-liquid-phase bonding has a limit in achieving small diameter/high-density bumps due to the extrusion of the melted Sn layer during bonding. This paper report a new method that can fabricate small diameter Cu-Sn bumps with 5μm diameter and 25μm pitch based on a new thermal reflow and pre-bonding method that enables solid-state Cu-Sn reaction for avoiding Sn extrusion. Using the new method, 3D integration of a 640 × 480 MEMS array on a CMOS circuit chip has been demonstrated. This method can be used in 3D integration of MEMS arrays and CMOS circuits such as micromirror arrays.