Fluxless Bonding of Silicon to Ag–Copper Using In–Ag With Two UBM Designs

A fluxless process of bonding silicon to Ag-cladded copper using electroplated In–Ag multilayer structure is developed. The Ag cladding on the copper substrate is a stress buffer to deal with the large mismatch in coefficient of thermal expansion (CTE) between semiconductors such as Si (3 ${hbox {ppm}}/^{circ}{hbox {C}}$) and Cu (17 ${hbox {ppm}}/^{circ}{hbox {C}}$). To manufacture Ag on copper substrate, two techniques are developed. The first is an electroplating process to fabricate a thick Ag layer. The second technique is a novel laminating process that bonds Ag foil directly on Cu substrate. On Si chips, two underbump metallurgy (UBM) structures are designed, Si/Cr/Au and Si/Cr/Ni/Au. To produce a solder layer, Si chips are electroplated with In followed by thin Ag. The thin Ag cap layer prevents oxidation of the inner In region. To achieve a fluxless feature, the bonding process is performed in a vacuum environment (50 mtorrs) to suppress indium oxidation. Compared to bonding in air, the oxygen content is reduced by a factor of 15 200. Using Cr/Au UBM structure, the silicon chip was detached from Cu substrate. The broken interface lies between Si/Cr and ${hbox {Ag}}_{2}{hbox {In}}$ IMC on Cu substrate. Using a new UBM design of Si/Cr/Ni/Au, high-quality joints are produced that comprise of three distinct layers of ${hbox {In}}_{7}{hbox {Ni}}_{3}$, ${hbox {Ag}}_{2}{hbox {In}}$ , and Ag. Microstructure and composition of the joints are studied using a scanning electron microscope (SEM) with energy dispersive X-ray spectroscopy (EDX).