Effective method to disperse and incorporate Carbon nanotubes in electroless Ni-P deposits

Abstract

The ever-increasing demand for higher I/O counts on chip requires the finer pitches to improve the performance, cost effectiveness and higher yield. The conventional under bump metallization (UBM) technology may not guarantee the required performance due to higher current density and diffusivity nowadays. In order to overcome these challenges, a UBM with higher strength and resistance to diffusion, better thermal and microstructural properties are required. Recently, nano reinforced Ni-P alloy has been identified as potential approach. Especially, the addition of Carbon nanotube (CNT) was expected to provide strengthening to the resulting UBM layers. CNTs have exceptional and attractive thermal, electrical, and mechanical properties, and are believed to be ideal material for fabricating composites. However, due to the large surface volume ratio, CNTs have strong tendency to agglomerate, which results in clusters. CNT clusters may lead to void formation and further cracking. Therefore, it is of great importance to figure out methods to disperse and incorporate CNTs into Ni-P UBM. In order to disperse carbon nano-tubes in electroless Ni-P coatings, two approaches were employed. Chemical modification, which includes acid oxidation treatment and surfactant treatment were conducted to disperse CNT clusters. Meanwhile, magnetic stirring and ultrasonic agitation were also employed to prepare the CNT/Ni-P composite coatings with CNTs homogeneously embedded. SEM and TEM were used to observe the dispersion after adopting the aforementioned treatment as well as the surface morphology of the deposited layers. It was verified that the dispersion can be significantly improved after proper treatment process, and surface of the CNT/Ni-P was quite smooth; CNTs were equably dispersed throughout the matrix. In addition, the interfacial bonding between CNTs and Ni-P coatings was good and firm. In these cases, an understanding of the effects of the acid oxidation, surfactant dispersant- magnetic stirring and ultrasonic agitation on CNT dispersion in both solution and deposited layer were ascertained.