Monolithic 3D-ICs with single grain Si thin film transistors

Monolithic 3D integration is the ultimate approach in 3D-ICs as it provides high-density and submicron vertical interconnects and hence transistor level integration. Here, high-quality Si layer formation at a low temperature is a key challenge. We review our recent achievements in monolithic 3D-ICs based on single-grain Si TFTs that are fabricated inside a single-grain with a low-temperature process. With the μ-Czochralski process based on a pulsed-laser crystallization, Si grains with a diameter of 6 μm are successfully formed on predetermined positions. Single-grain (SG) Si TFTs are fabricated inside the single-grain with mobility for electron and holes of 600 cm2/V s and 200 cm2/V s, respectively. Two layers of the SG Si TFT were vertically stacked and successfully implemented into CMOS inverter, 3D 6T-SRAM and single-grain lateral PIN photo-diode with in-pixel amplifier. Those results indicate that the SG TFTs are attractive for use in monolithic 3D-ICs on an arbitrary substrate including a glass and even a plastic for applications such as ultra-high-density memories, logic-to-logic integration, CPU integrated display, and high-definition image sensor for artificial retina.

► Monolithic 3D-ICs based on single-grain Si TFTs that are fabricated inside a single-grain with a low-temperature process.
► With the μ-Czochralski process, Si grains with a diameter of 6 μm are successfully formed on predetermined positions.
► Single-grain Si TFTs are fabricated inside the grain with mobility for electron and holes of 600 cm2/Vs and 200 cm2/Vs.
► Two layers of the SG Si TFT were stacked and implemented into inverter, SRAM and photo-diode with in-pixel amplifier.
► SG TFTs are attractive for use in monolithic 3D-ICs on an arbitrary substrate including a glass and even a plastic.