Growth of Cu6Sn5 and Cu3Sn Intermetallic compounds on (111)-, (100)-, and randomly-oriented copper films.

Abstract

Sn whisker has been found since 1940s.[1] The formation mechanism of whiskers was controversial. There were two possible mechanisms. Some scientists thought whisker formation were the result of recrystallization or abnormal grain growth, however others suggested that the atomic migration of Cu caused by compressive stress gradient were the main driving force of whisker formation. In latest study, Han-wen Lin et al. found that the whiskers growth mechanism is highly related with the growth of intermetallic compounds. The results showed that (100)- or (111)-oriented copper can slow down the growth of intermetallic compounds (IMCs) and whiskers growth. This can prove that the driving force of whiskers is compressive stress gradient produced by serious IMCs formation. As mentioned above, the growth of IMCs plays an important role in whisker formation, and copper are widely used as under bump metallization (UBM) so now we are interesting in the research of the growth mechanism of Cu6Sn5 and Cu3Sn IMCs. Our lab can fabricate highly (111)-oriented nano-twinned copper film and after annealing at appropriate temperature we can get (100)-oriented copper with large grain size. These copper films will serve as substrate. After certain duration of aging test, the results showed that the IMCs of copper and tin grew relatively fast along tin grain boundary. Also, we found that the growth of Cu6Sn5 and Cu3Sn was inhibited on (100)- and (111)-oriented copper films, and the grain of IMCs grew large in randomly-oriented copper film. So we suggest grain boundary diffusion of tin dominate IMCs formation in Cu/Sn system. Moreover, the IMC growth rate on (100)-oriented copper films are even slower than (111)-oriented copper films.