Microstructure and mechanical strength of snag-based solid liquid inter-diffusion bonds for 3 dimensional integrated circuits

Solid liquid inter-diffusion is a bonding technology that has been proposed at microscale for fabrication of ultrafine interconnects in high density and 3 dimensional integrated circuit packages. Due to very small size, bonds that are formed using this technology are very susceptible to process variations and thus reliability and quality of the bonds may deteriorate by small variations in the process. This study examines the effect of process parameters; temperature, bonding time, and pressure on the bond strength, microstructure, and intermetallic formation at the bonds. A full factorial experiment is designed and implemented on the process. Bonds are characterized using a micro-tester for mechanical strength and ductility. The microstructure of bonds formed under different process conditions are explored using scanning electron microscope, energy dispersive X-ray spectroscopy, and X-ray diffraction pattern to determine the bond thickness, elemental composition, copper concentration, and extent of copper diffusion and intermetallic compound. 1-dimensional phase lag modeling technique was used to estimate the time required for complete bond transformation to intermetallics. Results of these analyses show that temperature plays the strongest role in intermetallics formation and thickness. Bonds formed at higher temperatures and longer bonding time behaved more brittle and had higher strength. Pressure does not show a significant effect, and bonds that are formed under higher pressure show a slight reduction in bond strength with the same brittle behavior. This reduction is associated with formation of ε phase of intermetallic and the lower strength between ε and η phase. Modeling intermetallics formation for different bond thicknesses show that smaller bond thickness does not translate into much shorter time for complete transformation to intermetallics.