Although flexible memories have been developed, the devices reported thus far have large feature sizes and poor performance compared to conventional silicon wafer-based memories. This is due to material limitations and the limited fabrication methods feasible for use with plastic substrates. KAIST researchers got around these limitations by successfully transferring single-crystal silicon nanowire gate-all-around (GAA) SONOS memory devices onto a plastic substrate. They had equivalent erasing speeds, retention and endurance characteristics (>104 program/erase cycles) before and after the transfer onto plastic. Moreover, their electrical performance was maintained even when they were bent, opening the door to high-performance flexible products. The transfer process begins with a protective layer coating on the GAA SONOS devices, followed by their attachment to a temporary handle wafer. The backside is thinned until a buried oxide (BOX) layer is exposed. The BOX layer acts as an etch stop layer for the silicon. The remaining ultrathin (~1 μm) film layer consisting of the GAA SONOS devices is transferred onto a flexible film, followed by the removal of the temporary handler and the protection layer.

(a) is a photograph of the as-fabricated GAA SONOS FETs on an SOI wafer; (b) is a photograph of the ultrathin GAA SONOS devices transferred onto a flexible substrate. (a) Process flow of the fabrication of the GAA SONOS devices, along with a 3-D schematic diagram before interlayer dielectric deposition; (b) a TEM image of the GAA SONOS along the xx’ direction in (a); (c) a TEM image of O/N/O (3/6/13 nm) layers for the nonvolatile memory operation.

(Paper #27.6, “Flexible High-Performance Nonvolatile Memory by Transferring GAA Silicon Nanowire SONOS onto a Plastic Substrate,” J.-M. Choi et al, KAIST)

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