A Study on Improvement and Extension of Fine-Pitch Micro-Bump Interconnects Technology: New Metallurgy & Flux-Less Oxide-removal Laser Assembly (FLOLA)

Heterogeneous integration (HI) and advanced packaging technology have been taken notice of semiconductor industry because it was believed as an alternative to overcome the slowing-down of Moore’s law, by splitting a chip into a few ones and then integrating them again. Especially, hybrid copper bonding (HCB) was considered an ideal methodology because it could integrate such chiplets having extremely high interconnect density. However, there were some difficulties like sensitive copper dishing and particle control, and those challenged a quick settlement of this promising technology. Therefore, it will be helpful to share these challenges by improving and extending micro-bump technology. This paper explored the thermal stability of newly applied micro-bump metallurgies with a noble flux-less bonding process to improve and extend conventional micro-bump technology. Micro-bumps of 14 μm pitch were electroplated on a 77 mm x 77 mm coupon silicon wafer, and Flux-Less Oxide-removal Laser Assembly, FLOLA is applied to interconnect 11 mm x 11 mm chips on a coupon. Before the bonding process, Ar/H
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atmospheric plasma pre-treatment was carried out to remove tin-oxide on the surface of SnAg solder and, there was neither fluxing nor cleaning. Underfill encapsulation followed and a high-temperature storage (HTS) test was carried out to verify diffusion behavior for 14 μm pitch micro-bumps. New metallurgy micro-bumps (M-SnAg), consisting of a new metal pillar (M) and diffusion barrier, showed superior thermal stability and much lower intermetallic compound (IMC) growth compared to both Cu-SnAg and Ni-SnAg bump, even after 2000hrs of iso-thermal aging at 180°C.