Semiconductor crystal islands for three-dimensional integration

The critical operation needed to achieve monolithic three-dimensional integrated circuits is obtaining single-crystal, device-quality semiconductor material for upper layer active circuits without exceeding the thermal budget of underlying devices. Previous attempts at using pulsed laser crystallization of amorphous films for upper layer devices failed to provide large enough single crystals at a low enough temperature. Here, the authors demonstrate a more direct approach to realizing high-quality, single-crystal Si (100) and Ge (100) islands (3–3000 μm across) on amorphous SiO2 substrates. The technique is a form of hydrophilic fusion bonding featuring low temperatures (≤400 °C), chemical mechanical polishing, and chemical surface activation (using NH4OH). The bonds are strong enough (>1 J/m2) to withstand SmartCut® removal of the donor wafer. By bonding arrays of islands rather than one large contiguous layer, the authors effectively avoided the formation of thermally induced voids at the bonding interface, and so dramatically improved yield.