Low-temperature Al–Ge bonding for 3D integration

Low-temperature aluminum–germanium (Al–Ge) bonding has been investigated for monolithic three-dimensional integrated circuit (3DIC) applications. As upper layer devices of a monolithic 3DIC are fabricated in situ, a suitable technique for providing high-quality semiconducting material without inflicting damage to underlying circuits below is needed. Here, the authors demonstrate a method of attaching high-quality single-crystal Si (100) and Ge (100) islands (3–3000 μm in size) onto amorphous SiO2 substrates using both eutectic (435 °C) and subeutectic (400 °C) Al–Ge bonding. The 30 min, 3DIC compatible process utilizes Al–Ge bilayer films as thin as 157 nm to form void-free bonds strong enough to withstand SmartCut® hydrogen splitting of the donor wafer. The fracture energy of the Al–Ge bond was measured to be GC = 50.5 ± 12.7 J/m2, as measured by the double cantilever beam thin-film adhesion measurement technique.