Study of fine pitch micro-interconnections formed by low temperature bonded copper nanowires based anisotropic conductive film

In this paper, Cu nanowire based anisotropic conductive film (ACF) has been systematically investigated in terms of its fine-pitch potential, electrical/ mechanical properties, process related issues and failure mechanisms. A test chip module including daisy-chain and 4-point structures was fabricated by electroplating and photolithography process, which has 160, 80, 40 and 30 μm bond pad pitches. The selection of membrane templates and the template based open/short failures have been discussed. Bond pads were finished with electroplated In to lower the bonding temperature and contact resistance. The electrical and mechanical performance of the interconnections has been studied in term of bonding force. The interconnect resistance of various pad sizes was 0.3-0.6 Ω per pad at the low bonding force of 1.5 N and dropped to 0.02-0.04 Ω per pad at the high bonding force of 10 N. The influence of bonding pressure on such nanowires formed interconnects was studied by micro-sectional analysis. For all pitch sizes, the electrical insulation was maintained for an applied voltage of 20 V. A shear strength of 1-5 MPa was achieved as a function of the bonding force and the fractural surface analysis verifies the Cu-In joint formed in such interconnections.