Application of self-assembled monolayer (SAM) in low temperature bump-less Cu-Cu bonding for advanced 3D IC

Self assembled monolayer (SAM) of alkane-thiol of 6-carbon (1-hexanethiol, C6) chain length is applied on Cu surface (deposited on Si substrate) and examined carefully. Firstly, the ability of SAM adsorption onto Cu surface is confirmed by the sharp rise of water contact angle (CA) on the surface. Next, the thermal stability of SAM when it is stored in different environments is studied. The CA decreases when it is stored in clean room ambient due to partial desorption of the SAM. The desorption behavior of SAM is found to be reversely proportional to the immersion time in SAM solution, whereby longer immersion time shows less desorption. SAM desorption can slowed down significantly if samples are kept at lower temperature (~4°C). Substantial desorption of SAM is observed when the samples are annealed above a critical temperature when SAM desorb rapidly from the Cu surface. Surface analysis confirms that Cu surface protected by SAM contains less oxygen. Finally, bonding experiments are performed to validate effectiveness of SAM in tailoring the Cu surface for bonding enhancement at low temperature. Results show that uniform Cu-Cu bond with higher shear strength is obtained as a result of SAM passivation.