Application of self assembly monolayer (SAM) in Cu-Cu bonding enhancement at low temperature for 3-D integration

In our work, self assembled monolayer (SAM) of alkane-thiol of 6-carbon (1-hexanethiol, C6) and 12-carbon (1-dodecanethiol, C12) chain length are applied on Cu surface (deposited on Si substrate) and examined carefully. Firstly, the ability of SAM adsorption onto Cu surface is confirmed by the reduction of the surface roughness and the sharp rise of water contact angle (CA) on the surface. Next, SAM stability when it is subjected to various exposure times in ambient environment and annealing in inert ambient are also studied. The CA for both cases decreases as compared to freshly prepared SAM samples. This indicates that SAM degradation or desorption has occurred during exposure to ambient environment and annealing. The desorption behavior of SAM is found to be reversely proportional to the immersion time in SAM solution, whereby longer immersion time shows less degradation and desorption. C12 exhibits a higher resistance to degradation and higher thermal stability as compared to C6 under the above conditions as seen from a slower drop in CA. This is due to the fact that longer chain length has a denser molecular arrangement and hence stronger inter-molecular force which results in better thermal stability. Next, SAM formation is further analyzed by x-ray photoelectron spectroscopy (XPS) to verify its adsorption and desorption properties. It is shown that SAM is able to retard metal oxide formation. The degree of retardation of oxide formation is also found to be proportionate with longer immersion time. The results show that SAM is able to retard copper surface oxidation in the air and the degree of SAM desorption through annealing is proportionate to annealing time and is temperature highly dependent. Finally, bonding experiments are performed to validate effectiveness of SAM in tailoring the Cu surface for bonding enhancement at low temperature.