Unidirectional Growth Of Cu-Sn Compounds Assisted By Electric Currents And Ultrafast Bonding Mechanism For Microsolder Joints

The microelectronic devices used in the areas like information,energy and space exploration are experiencing unprecedented growth due to their rapid multi-functionalization,miniaturization as well as highly reliable requirments.The threedimensional?3D?electronic packaging technology as a key direction in the next generation information technology not only can downsize the dimension but also improve the performance of microelectronics.Futhermore,the size of interconnection was decrease from a few centimeters to several micrometers,meanwhile the corresponding bonding method was changed dramatically.Recently,the joints in the 3D stacking chips completely consisted of intermetallic compounds?IMCs?have been drawn extensive attentions.However,the fabrication for the full IMCs joints was highly time-consuming during the conventional solder reflowing processes,and the bonding pressure was too high,which would seriously affect the reliability of the packaging system.Therefore,it is a burning question for fabricating full IMCs joints rapidly and efficiently for the application in 3D stacking chips.The present doctoral thesis investigates the ultrafast fabrication of IMC joints using an electric current-assisted bonding process at ambient temperature based on the joule heat effect and the electron wind force-induced solid-liquid electromigration?S-L EM?.By carefully controlling the current densities?¹04 A/cm2?,the expected temperatures corresponding to different IMC microstructures are achieved within a very short duration?^ms?.This extremely fast and low cost approach offers full control of the formation and unidirectional growth of IMCs,which is a promising tool for interconnection in 3D integration circuits.According to the experiment results and theory analysis,the ultrafast electric current-assisted bonding mechanism,the microstructure evolution rules of interfacial IMCs,the S-L EM at the interface of Cu/molten Sn,unidirectional growth principle of IMCs under the electric current stressing as well as the assessment of mechanical property of as-fabricated solder joints were extensively investigated.Based on the structure features of 3D stacking chips,two electric currentassisted bonding modes,parallel-electrode and butted-electrode type,were designed respectively.After the optimizing the process parameters such as bonding pressure,time,current densities,thickness of Sn interlayer and so on,the high-temperaturestable Cu₃ Sn joints without voids were selectively fabricated within an extremely short time?¹80 ms?and under a low pressure of 0.16 MPa in a Cu/Sn/Cu interconnection system at ambient temperature.Meanwhile,three different types of full Cu-Sn IMCs joints were selectively fabricated using electric current-assisted bonding process in butted-electrode type.By mean of selectively deep-etching procedure,the three-dimensional morphologies of interfacial IMCs after electric current-assisted bonding process were studied,at the same time the microstructure evolution rules were summarized.due to constitutional supercooling,the cell-shape Cu₆Sn₅ formed at the initial stage was changed into cellular dendrites then into columnar dendrites.When the joule heatinduced temperature increased above the melting point of Cu₆Sn₅,the initially formed dendritic Cu₆Sn₅ grains were melted and transformed into dendritic Cu₃ Sn until the residual Sn solder were completely consumed.The computed results of Cu atoms amounts at anode side using MATLAB software indicated that S-L EM at the high joule heat-induced temperature can furnish sufficient Cu atoms for forming Cu₃ Sn joints.The highly unidirectional [100] growth of Cu₃ Sn IMCs was achieved along the direction of electron flow.By calculating the planar atomic densities of projected images on different planes,the particular growth direction was confirmed to represent the low-scattering path for the traveling electron flow.Furthermore,the shear tests showed that the type,morphology and volume proportion of interfacial IMCs can strongly influence the shear strength of those as-fabricated joints.The shear strength of the as-fabricated joints was both increased with the boinding time and current density.The oriented Cu₃ Sn joints exhibited the most excellent shear strength of 61.0⁷1.1MPa,which was approximately 2.5 times higher than the Sn-based solder joints and 1.8 times higher than the solder joints with dendritic Cu₆Sn₅.The S-L EM tests at relatively low temperature with the current densities at the level of 102 A/cm2 demonstrate that the polarity effect of interfacial IMCs was obvious at the initial stage of current stressing.The S-L EM can enhance the Cu₆Sn₅ growth at anode side while hinder that at cathode side.the interfacial IMCs growth kinetics analysis indicate that the S-L EM of Cu solute atoms was dominant to the epitaxial growth of Cu₆Sn₅ at the anode side,compared with the grain boundary diffusion and volume diffusion at the Cu/molten Sn interface.In addition,the grain orientation analysis indicated that the epitaxial growth Cu₆Sn₅ formed at the initial stage grew in [0001] direction with respect to the electron flow.The [0001] growth direction of Cu₆Sn₅ represented the low-resistance paths for electron flow.Moreover,the tendency of the unidirectional growth become more obvious with the increase of the applied current densities.