Co-electroplating Sn—Bi alloy solder for 3D-IC low-temperature bonding

An electrolyte bath solution for co-depositing tin (Sn) and bismuth (Bi) comprising:

• Bi3+ ions released from a bismuth salt into the electrolyte bath solution;
• Sn2+ ions released from a tin salt into the electrolyte bath solution;
• a first reagent that is selected from the group consisting of a nitric acid, a sulfuric acid, a carboxylic acid, a sulfonic acid, and a phenyl acid;
• a second reagent that is selected from the group consisting of an imine, a ketone, an aldehyde, and a thiophene;
• wherein the first reagent and the second reagent form a complex with the Bi3+ ions in the electrolyte bath solution;
• and
• a third reagent having a third reagent molecule that is an organic molecule having a hydrophilic end and a hydrophobic tail;
• wherein the hydrophilic end has carbon atoms and hydrogen atoms and at least one heteroatom that is not carbon or hydrogen, wherein the hydrophilic end attaches to a surface of a cathode immersed in the electrolyte bath solution;
• wherein the hydrophobic tail is an aliphatic chain having only carbon and hydrogen atoms, the hydrophobic tail attracting hydrogen gas generated at the surface of the cathode;
• wherein the hydrophobic tail comprises a contiguous group of at least 17 carbon atoms;
• wherein the at least one heteroatom in the hydrophilic end is an oxygen atom, a nitrogen atom, or a sulfur atom, and the hydrophilic end has no more than 5 oxygen atoms;
• and
• wherein the third reagent molecule is synthesizable by a reflux reaction of a polymer molecule with two amine ends and two chlorinated hydrocarbon molecules, each chlorinated hydrocarbon molecule having a side branch between a long aliphatic chain and a chlorine end,
• whereby the hydrophobic tail of the third reagent molecule attracts hydrogen gas to remove hydrogen gas from near the surface of the cathode to permit uniform co-deposition of Bi3+ ions and Sn2+ ions on the surface of the cathode without hinderance from the hydrogen gas.