Achieving low temperature Cu to Cu diffusion bonding with self assembly monolayer (SAM) passivation

In this article, we investigate the feasibility of applying a self assembly monolayer (SAM) onto Cu surface as a passivation layer so as to lower the required temperature during Cu-Cu bonding. Proprietary SAM is applied on Cu layer deposited on Si wafers and studied carefully. The stability of SAM when it is subjected to various exposure times in ambient air and high temperature anneal in inert environment are examined. The contact angle reduces in both cases as compared to freshly coated SAM suggesting that SAM degrades or desorbs over time in room ambient and during high temperature annealing. It is found that the degradation or desorption degree of SAM is inversely proportional to the immersion time in SAM solution. Bonding experiments are carried out to verify the effectiveness of SAM passivation in bonding enhancement at low temperature. The application of SAM passivation layer on Cu layer in order to realize low temperature diffusion bonding is studied. It is found that SAM can effectively passivate Cu surface and readily desorbed prior to bonding. This can provide an ultra-clean Cu surface to achieve diffusion bonding at lower (< 300degC) temperature.