Application of self assembly monolayer (SAM) in lowering the process temperature during Cu-Cu diffusion bonding of 3D IC

In this research work, we investigate the feasibility of lowering the bonding temperature by refining the copper surface with the application of a self assembly monolayer (SAM). First, the stability of SAM when it is subjected to different exposure environments is measured quantitatively by the change in the contact angle. The different environments includes: 1) exposure in clean room ambient; and 2) high temperature annealing. The contact angle is reduced for these two cases suggesting that SAM degrades or desorbs over a period of time and during high temperature annealing. Next, the formation of SAM is further analyzed qualitatively by X-ray photoelectron spectroscopy (XPS) to verify its adsorption and degradation properties. The result shows that SAM is able to slow down the oxidation process on copper surface. Bonding experiments are then performed in order to confirm the effectiveness of SAM in achieving a low temperature bonding. The formation of SAM has been shown useful in protecting copper surface from oxide formation; and at moderate temperature annealing (<300°C), it readily desorbs prior to bonding. Surface passivation with SAM can therefore help to lower the bonding temperature by provide a clean and a larger contact area for successful bonding.