Low-temperature Al–Ge bonding for 3D integration
(Color online) (a) AFM scan (5 × 5 μm) of as-deposited bilayer with surface roughness of 1.63 nm RMS; (b) FIB cross-section of evaporated Al–Ge stack on top of SiO2 diffusion barrier. Note: Ge surface roughness in (b) is due to Ga+ ion milling process (FIB).
Schematic of the DCB thin-film adhesion measurement technique. The thin films (or bond layer) reside between two linear elastic beams, which are loaded as cantilevers.
(Color online) SEM plan-view image and the corresponding AES elemental map of Al–Ge eutectic bond formed at 435 °C. The top wafer has been ground and etched away to reveal the eutectic binary alloy segregated into germanium dendrites within an aluminum matrix.
TEM cross-section of Al–Ge eutectic bond formed at 435 °C. The void-free bonding layer consists of laterally segregated Al and Ge domains that span the full thickness of the layer (approx. 175 nm), as indicated by the EDS spectra (not shown).
(Color online) (a) Wafer flat area of 4-in. oxidized Si wafer pair bonded at 450 °C contains a small drop of solidified Al–Ge alloy that has been squeezed out from the interface during the bonding step, providing further evidence that Al–Ge eutectic melt occurred; (b) SEM image of the drop from (a) confirms the typical morphology of solidified Al–Ge eutectic alloy.
SEM cross-section of sub-eutectic Al–Ge bond at 400 °C. The mated wafers had 475 nm thick Al–Ge bilayer each and were bonded for 2 h at 350 kPa down-pressure. Even though no melt occurred, clear segregation into Al and Ge rich domains is visible. The original bonding interface is still discernable, with a few sub-micrometer voids present.
(Color online) Al–Ge bonding of Si islands for monolithic 3DICs: (a) thermally oxidized Si (100) donor wafer is implanted with H+ ions (6 × 1016 cm−2, 75 keV, Rp = 630 nm) for eventual SmartCut; (b) donor islands are patterned and dry-etched; (c) Al–Ge bilayer is evaporated onto an oxidized Si acceptor wafer and patterned into pads; (d) donor and acceptor wafers to be bonded are pressed face-to-face to form a Al–Ge eutectic bond; (f) the donor wafer is split away via SmartCut process; (g) the resulting surface roughness is removed with CMP touch polish (h), furnishing single crystalline Si (100) islands on top of amorphous SiO2 acceptor wafer.
(Color online) (a,b) SEM images of Si (100) islands and (c) optical image of Ge (100) island attached to SiO2 acceptor wafer via sub-eutectic (a,c) or eutectic (b) AI-Ge bond. The donor wafer has been successfully removed via SmartCut, and the islands are ready for the CMP touch polish. Note: Bright region in (c) is Al-Ge bonding pad not covered by the island.
(a) Transmission infra-red (IR) image of the Al–Ge bonded 4-inch wafer pair showing a uniform array of islands; (b) SEM image of large Si (100) islands transferred via sub-eutectic Al–Ge bonding at 400 °C onto the SiO2 substrate; (c) SEM micrograph depicting incomplete transfer of a Si crystal island due to hydrogen exfoliation (SmartCut) taking place during the Al–Ge eutectic bonding step at 435 °C.
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