8. Monolithic 3D Chip
Category: 3D Devices and Circuits
Paper 25.4 - Low-Cost and TSV-free Monolithic 3D-IC with Heterogeneous Integration of Logic, Memory and Sensor Analogy Circuitry for Internet of Things; Tsung-Ta Wu et al, National Nano Device Laboratories/National Tsing Hua University
3D circuits often are made by stacking separate chips and connecting them electrically with through-silicon vias (TSVs), but TSVs have significant disadvantages including relatively narrow I/O bandwidth. Monolithic 3D ICs with no TSVs—where the devices in adjacent layers are directly connected—have been demonstrated, but transistor damage from thermal annealing can arise. That’s because each layer in a 3D device must be annealed to remove stresses in its crystalline silicon structure, and also to activate the dopants which have been implanted in it. However, the high heat involved with annealing (>1,000°C) can damage the devices that have already been built in lower layers. A team led by Taiwan’s National Nano Device Laboratories addressed this issue by using a CO2 far-infrared laser at 400°C to selectively pulse-anneal specific areas of the silicon (the source-drain regions). They used this technique to build a sub-40nm monolithic IC containing a variety of heterogeneous functions—logic, SRAM, RRAM, sense and analog amplifiers, and gas sensors. No device degradation was reported, and the researchers say their technique is suitable for making the low-power, low-cost, small-footprint and heterogeneously integrated devices needed for the Internet of Things.