Low Temperature Fine Pitch All-Copper Interconnects Combining Photopatternable Underfill Films

The trend to 3D and heterogeneous integration enable driving multi-functional blocks in one package. Flip-chip integration is currently playing an important role and is based on solder joints. To overcome the limitations of solder joints, all-copper interconnects have been investigated to meet electrical, thermal, and reliability demands in 3D integration. The underfill process is widely applied in flip-chip encapsulation technology. We propose a novel wafer-scale all-Cu interconnect method combining epoxy-based photo-patternable polymer as self-aligned underfill layer with the patterned copper nanoparticles interconnects. The resulting test wafers were able to pattern 20 μm pitch copper nanoparticle-paste interconnects on both substrates with and without photoimageable polymer. The Cu paste was applied to form the interconnects and was sintered after bonding process. Free-standing nanocopper is sintered to obtain mechanical properties with a Young's modulus of 112 GPa. All-Cu interconnects with diameter of 50 μm and 100 μm were measured to achieve the specific contact resistance, ranging from
$1.4 \times 10^{-5}\Omega\cdot \text{cm}^{2}$
to
$1.0 \times 10^{-5}\Omega\cdot \text{cm}^{2}$
at different sintering temperature when epoxy-based underfill existing. And its resistivity was
$4.54\times 10^{-4}\Omega\cdot \text{cm}$
, compared to
$5.86\times 10^{-4}\Omega\cdot\text{cn}$
for the all-Cu interconnects without underfill.