Scalable, sub 2μm pitch, Cu/SiCN to Cu/SiCN hybrid wafer-to-wafer bonding technology
This paper presents a novel approach to face-to-face wafer-to-wafer (W2W) bonding using SiCN-to-SiCN dielectric bonding, in combination with direct Cu-Cu bonding using Cu pads of unequal size and surface topography for the top and bottom wafers. The use of SiCN dielectrics allows to obtain a high W2W bonding energy (> 2 J/m2) at low annealing temperature (250 °C). Excellent Cu-Cu bonding is obtained after annealing at 350 °C. A novel CMP process, resulting in a slightly protruding Cu top pad and a slightly recessed Cu bottom pad, is introduced. The difference in pad sizes, allows for the necessary W2W overlay bonding tolerances. Excellent resistivity and yield results are obtained across bonded 300 mm Si wafers for scaled 360 nm top pads bonded to 720 nm bottom pads at 1.44 μm pitch (25% bottom Cu density). Feasibility of smaller pitches has been demonstrated by successfully bonding 180 nm top pads to 540 nm bottom pads at 0.72 μm pitch.
"Hole-in-One TSV", a New Via Last Concept for High Density 3D-SOC Interconnects
This paper presents face-to-face wafer-to-wafer (W2W) bonding using SiCN-to-SiCN dielectric bonding, in combination a novel 1µm diameter via last connection between top and bottom wafers, called "hole-in-one TSV". This scheme reduces besides the interconnection pitch and also the number of processing steps. The hole-in-one TSV is introducing an innovating integration modification. With the introduction of a cavity etched in the top wafer prior to W2W bonding, the via last etch process is simplified. The etch time of the dielectric part of the TSV etch is heavily reduced, minimizing the over etch time on the top landing metal to open the landing wafer's metal pad. Critical integration steps like CMP processes, wafer bonding and thinning to 5µm Si thickness are highlighted, together with alignment tolerances and connectivity yield between the bonded 300mm Si wafers.
Phase characterisation of simulated high burn-up UO2 fuel
Abstract To determine the lattice constant variation with burn-up and the oxidation behaviour at high temperatures, simulated UO2-spent fuels with burn-ups up to 200 GWd tM−1 were prepared by cold pressing and sintering and characterised by X-ray diffraction analysis and thermogravimetry. The fcc-lattice parameter of the sintered samples determined at room temperature, showed a continuous decrease with the simulated burn-up. During oxidation the conversion of the simulated fuels to the (U,FP)4O9 and (U,FP)3O8-type phases was strongly delayed when the simulated burn-up exceeded 80 GWd tM−1. Also, at high burn-ups (>100 GWd tM−1), large fractions of the (U,FP)4O9-type phase remained untransformed to (U,FP)3O8 after prolonged exposure to air at 950–1000°C. This suggests a marked widening of the (U,FP)4O9-phase field when the amount of substituted uranium atoms exceeds values of about 8%.