Enhancing Cu-Cu Diffusion Bonding at Low Temperature Via Application of Self-assembled Monolayer Passivation

Abstract

Self-assembled monolayer (SAM) of alkane-thiol is employed to passivate Cu surface in an attempt to preserve its surface cleanliness. Alkane-thiol molecules can self-assemble readily onto the Cu surface forming a densely packed monolayer. SAM formed on Cu surface is found to be able to retard surface oxidation and the degree of protection can be enhanced by alkane-thiol with longer carbon chain length. SAM formed is prone to partial degradation in ambient air and can be desorbed by thermal annealing at moderate temperature (<300°C). SAM passivated Cu wafers are bonded after a pre-bonding anneal step to desorb the SAM. The bonded wafers show enhanced bonding uniformity and efficient Cu cross-diffusion compared to control wafers when shorter chain length of alkane-thiol molecule (n = 6) is used. SIMS analysis shows that there are carbon and oxygen impurities pile-up at the bonding interface when longer chain length alkane-thiol (n = 12) is used, as a result of incomplete desorption prior to bonding. With the selection of the appropriate chain length, clean surface can be conserved after desorption in inert ambient and hence promoting grain growth during Cu bonding. Mechanical shear measurement is performed and enhancement in strength with SAM passivation is confirmed.