Material selection for hybrid floating gate NAND memory applications

This article discusses the materials aspects involved in the successful integration of hybrid floating gate (HFG) devices for NAND applications. In HFG, a Si(n type)\metal(p type) stack replaces the standard poly-Si FG. The high work function metal helps to enlarge the program window by limiting the leakage through the high-k intergate dielectric (IGD); the use of high-k IGD enables to scale the equivalent oxide thickness (EOT) of the HFG cell. Our recent progresses in the understanding of the materials characteristics (IGD crystallinity and HFG-IGD thermal stability) that influence memory performance have allowed to demonstrate that HFG can be a solution to scale planar flash beyond the 20 nm node with the best preferable hybrid floating gate structure being a Ru-based HFG and a 3-layers IGD stack of HfAlO\Al2O3\HfAlO. Indeed, through the correct selection and combination of materials for process conditions that are relevant in flash memory fabrication flows, we will show that program windows can be suitably engineered. Finally, the materials challenges for keeping acceptable memory retention characteristics are also highlighted.