Wafer-to-wafer bonding is an essential process step to enable 3D stacked integrated circuits and the copper/oxide hybrid bonding process is a key enabler for such applications. It was demonstrated in Leti's cleanrooms using EVG's fully automated GEMINI FB XT fusion wafer bonding system, at a pitch of 1µm with copper pads as small as 500nm.
This result was obtained in the framework of the program IRT Nanoelec headed by Leti. EVG joined the institute's 3D Integration Consortium in February 2016. In order to shrink IC dies, tight alignment and overlay accuracy between the wafers is required, not only to minimize the interconnect area at the bond interface but also to ensure good electrical contact between the interconnected device on the bonded wafers.
In the Leti demonstration, the top and bottom 300mm wafers were directly bonded in the GEMINI FB XT automated production fusion bonding system, which incorporates EVG's proprietary SmartView NT face-to-face aligner and an alignment verification module to enable in-situ post-bond IR alignment measurement. The system achieved overlay alignment accuracy to within 195nm (3-sigma) overall, with mean alignment results well centered below 15nm. Post-bake acoustic microscopy scans of the full 300mm bonded wafer stack as well as specific dies confirmed a defect-free bonding interface for pitches ranging from 1µm to 4µm with optimum copper density.