Chemical mechanical polishing for indium bond pad damascene processing

We investigated chemical mechanical polishing (CMP) of indium, with the goal of obtaining indium bond pads for later cryo-3D integration of quantum computing-related chips, through bonding between these bond pads and indium bumps. Higher removal rates were obtained with soft CMP pads than with hard pads. The latter led to deep scratching, while this effect was much more limited for soft pad CMP. On patterned wafers, indium is cleared well in structured areas using soft pad CMP, leading to relatively high-quality indium surfaces inside bond pads, although corrosion might be of some concern. Pattern density uniformity was an important factor for within-die deviation in indium clearing time. Dishing was much more limited than in earlier work on indium polishing, while surface roughness was also found to be relatively limited. The obtained indium damascene bond pads may be suitable for 3D die-to-die and wafer-to-wafer bonding through indium pad-to-bump bonding.