High throughput Cu-Cu bonding by non-thermo-compression method

A non-thermo-compression bonding method by ways of a permanent surface passivation is applied and activated to allow instantaneous Cu-Cu bonding in room ambient hence improving the throughput by at least 30×. This permanent surface finish layer is ultra-thin, CMOS compatible, electrically conductive, resistant to oxidation, does not intermix with Cu, and can be activated readily to achieve fusion bonding (hence not diffusion limited). In this work, clean bump-less Cu surface (100 nm) is coated with a thin passivation layer of 10 nm and protected against ambient oxidation or corrosion. Prior to bonding, the surface is cleaned and activated to achieve a hydrophilic surface with contact angle ~ 10° and RMS roughness of 0.35 nm. A pair of wafers is then brought into intimate contact and bonding is achieved instantaneously via surface energy interaction in clean room ambient. Since the melting temperature of the passivation layer is much higher than the back-end temperature, it is possible to enhance the bond strength further by post-bonding annealing in a batch process. Experimental data show that the mechanical and electrical performance of the Cu-Cu bond is not deteriorated with the application of the permanent passivation layer. The insertion of the thin coating results in a slight increase in sheet resistance from 0.16 ohm/sq (Cu/Cu) to 0.18 ohm/sq (Cu/passivation layer/Cu). I-V measurement across the bonding interface of the non-TCB sample does not reveal clear departure from that of the TCB sample. Four point bending test is performed and the adhesion energy is 14 mJ/m² (non-TCB) as compared with 12 mJ/m² for TCB Cu-Cu. The bonding interface is seamless with no void as observed under TEM. This is a major improvement in throughput as compared to thermo-compression bonding as the bonding step is completed in several seconds upon contact without the need for prolonged compression and heating. In addition, bonding is performed in room ambient (25 °C, 1 ATM) with existing tool set. The need of inert bonding chamber is optional. Room temperature bonding is essential to avoid thermal run-out in alignment accuracy as well as better post-bonding thermo-mechanical stress control. Since this non-TCB bonding method is accomplished instantaneously upon contact initialization, it is possible to extend this method to die-on-wafer bonding with reasonable throughput. Die-on-wafer bonding offers greater flexibility in heterogeneous 3D IC stacking with different die size which is not possible by wafer-on-wafer bonding.