Advanced electronic interconnections based on template-assisted electrodeposited Cu nanowire/micropillar arrays

With the rapid development of electronics towards miniaturization and increased functionality, the heterogeneous integration of electronic components necessities a paradigm shift in the design of physical dispositions and selection of materials, as the conventional joining approach is becoming increasingly restrictive. Low temperature bonding is considered as one of the most favourable methods to enable advanced packaging with the uses of a wide range of intermediate materials, such as pastes, preforms and conductive adhesives. However, various drawbacks are commonly observed in the current practices with existing processes and materials. Therefore, the development of advanced interconnect technologies and novel materials is imperative to meet the ever-increasing demands. In this study, interconnection manufacturing routes have been developed with an attempt of exploiting the unique Cu nanowire/micropillar arrays produced by template-assisted electrodeposition (TAED).

In the present work, the Cu nanowire array (Cu NWA), Cu-Sn nanocomposite interlayer (Cu-Sn NI) and Cu micropillar array (Cu MPA) have been successfully prepared through TAED. The growth characteristics of these arrays or interlayers are examined to make them more tailorable in the subsequent bonding processes, by considering the effects of deposition parameters on the microstructures, morphologies, and process optimisation. 

In line with demands of three-dimensional integrated circuit (3D IC), the uses of purposely designed Cu-Sn NI are made in the experiments to accelerate the transient liquid phase bonding (TLPB) process, which is extremely slow based on conventional bonding process. The Cu-Sn NI consists of Cu NWA infiltrated with Sn, in which the nanowires are densely but separately distributed perpendicular to the surface of Cu substrates. Results show that this Cu-Sn NI can accelerate the TLPB process effectively by over 20 times. Moreover, the intermetallic compound (IMC) joints formed with this interlayer possesses refined equiaxed Cu6Sn5 grains which exhibits ~29.1% higher bonding strength than the joints bonded with Sn interlayer of the same thickness and bonding conditions. The underlying mechanism of such bonding process is therefore investigated, with an emphasis put on the elaboration of the formation of various defects and the potential mitigation solutions.

In order to supress the formation of voids and solder bleeding that are commonly observed in self-propagating exothermic reaction (SPER) bonding, the Cu-Sn NI is also utilised to replace conventional solder interlayers (e.g., Sn-based alloys). The bonded structures achieved under different preheat temperature and pressure are examined in terms of their microstructures and mechanical integrity. The research has found that the joints formed with Cu-Sn NI exhibit significantly improved bonding quality in comparison with those with the Sn solder layers of same thickness under the same bonding conditions. It has been proven that the uses of Cu-Sn NI are effective to reduce porosity and solder bleeding, which has resulted in a higher shear strength. The fundamental mechanism of such improvements has therefore been explicitly explained in connection with the unique properties of Cu-Sn NI during SPER bonding. 

A novel fluxless insertion bonding is also proposed and realized by pressing the Cu MPAs into solder bumps under a low process temperature. The morphologies, microstructures and shear strengths of the joints formed under different bonding parameters with Cu MPAs have been investigated in detail to understand the formation of the interfaces and joints. It is found that effective interconnections could be achieved even at 20 °C due to the mechanical interlock between the solder bumps and Cu MPAs. Joints formed with longer bonding time at higher temperature possess higher shear strength due to the formation of more IMC at the bonding interfaces. Moreover, the morphology of the Cu MPA is found to have significant influences on the shear strength of bonded joint. Sn coating is also found to be helpful in the formation of effective metallurgical interconnection, leading to the enhancement of bonding strength.

All aforementioned materials and bonding methods developed, with a wealth of research findings yielded in this work, may open a new window of innovative manufacturing routes with great potential for advanced electronic interconnections in the future industrial applications.