Fabrication of highly-uniform indium ball bumps for small unit-cell infrared focal plane arrays

Highly uniform ball-shaped indium bump arrays with small pixel pitches down to 10 μm have been fabricated. Multilayer stacked metal contact electrodes covered by a thin SiN_x dielectric layer serve as the under bump metallization. Indium bumps were thermally evaporated inside the SiN_x openings on top of the electrodes. Wet lift-off of the indium bumps was achieved by using a negative photoresist with precisely controlled undercuts. By comparison to a recipe without the SiN_x, the non-uniform reflow effect of the indium materials was effectively eliminated after the thermal treatment. A mean indium ball diameter of 6.05 μm with a small coefficient of variation of 2.6% was finally realized for 10 μm pitch arrays. These results demonstrate the fabrication method is promising to ensure a reliable flip-chip hybridization of ultra-fine pitch focal plane arrays to silicon readout circuits with high yield.