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

Abstract

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 5 μm diameter and 25 μm 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. © 2024 Institute of Electrical Engineer of Japan and Wiley Periodicals LLC.