Fluxless Bonding of Silicon to Molybdenum Using Sn-rich Solders

A fluxless bonding process between a 2" silicon wafer and a 2" x 2" molybdenum substrate with Sn-rich solder has been successfully developed. To achieve high quality joints, flux is avoided during bonding process. The flux or flux residues could be easily trapped in the joint, resulting in voids and uneven solder layer. Flux residues, if not completely removed, could also cause potential reliability problem. The key to carry out fluxless bonding is the oxygen-free environment during solder manufacture and bonding process. A thin Ag capping layer is electroplated immediately over the Sn layer after a thick Sn layer is electroplated on the Mo substrate with Cr/Au structure. This outer Ag layer prevents the inner tin from oxidation when the sample is exposed to air. The electroplated Mo substrate with Cr/Au/Sn/Ag structure is then bonded to a silicon wafer with Cr/Au structure in vacuum environment to suppress tin oxidation. Therefore, no flux is used. Microstructure and composition of the joints are studied under scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX). Scanning acoustic microscopy (SAM) is also used to verify the quality of joints over the entire sample. Nearly void-free joints are achieved and the composition of the joints is more than 97 at. % Sn. The Sn-rich solder helps release the stress in a bonded pair and improves reliability of joints. The success of this development shows that it is indeed possible to bond an entire wafer to a different material by Sn-rich solder without using any flux.