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A Tin-Silver Alloy Electroplating Process for Flip Chip Interconnect
Keywords: tin-silver SnAg, lead-free Pb-free , wafer bump
Tin-silver alloy electroplating has become the industry's choice for fine pitch flip-chip interconnect technology to replace tin-lead driven by environmental considerations and government restrictions on the use of lead. One of the challenges to electrodeposit an alloy that consists of an inert metal and a more active metal is bringing the standard deposition potentials close together by using the proper complexing agent, so that the two metals can codeposit simultaneously and at the desired alloy composition. Furthermore, it is also essential to achieve a stable alloy composition over a wide operating window and across the wafer surface. The present work describes a new methanesulfonic acid based, high-speed, tin-silver alloy electroplating process which is capable of plating a SnAg alloy with 2-3% wt silver over a wide operating window. The morphology of the SnAg deposit is smooth, compact and free of nodules. Photoresist patterned 200mm wafers with 125 m x 50 m vias were plated with mushroom bumps under a variety of operating conditions. From profilometer measurements, both with-in-die and with-in-wafer thickness uniformities were below 10% and coplanarity was below 15%. Silver composition across a 200mm wafer was within 2-3% as measured by Atomic Absorption Spectroscopy (AAS). SnAg bumps were reflowed and X-ray inspected. Void free bumps were achieved across a wide current density range of 4 to 8 A/dm2. The tin-silver electrolyte is very stable and all the components are analyzable. Data will be presented from work conducted in production tool sets.
Yu Luo, Research Chemist
Rohm and Haas Electronic Materials, LLC
Marlborough, MA

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