Flip-chip bonding technique using indium bump for hybrid IRFPA(Infrared Focal Plane Array) is studied. The mismatch in the coefficients of thermal expansion between HgCdTe and silicon is related to flip-chip bonding reliability. To improve flip-chip bonding reliability, strong adhesion between indium bump and contact pad is required.
Increasing bonding temperature over melting point of indium(156℃) is easy way to achieve strong adhesion between indium bump and contact pad, but HgCdTe wafer would be damaged. To minimize the damage of HgCdTe wafer and to maximize adhesion, contact pad is changed to indium-bismuth alloy which has low melting point(71℃) and adhesive to indium.
Increasing bonding temperature affected good adhesion between indium bump and indium-bismuth pad but still has leveling problem related to flip-chip bonder. To avoid this problem, post bonding reflow is necessary. It is shown that post bonding reflow around 120℃ has an effect on bonding reliability, both electrical and mechanical property.
