A Study on Electrical Characteristics in 3D NAND Flash Memory with a Poly-Si Channel

Abstract

Currently, several 3D NAND flash memories have been developed and commercialized. In terms of process, as the production of 3D NAND flash memories has considerably fewer lithography steps compared to 2D NAND flash memory, it has the advantage of bit density and innovative bit cost reduction. However, 3D NAND flash memories have additional considerations for electrical characteristics because most 3D NAND flash memories use poly-Si as the channel material instead of the conventional single-crystalline silicon(c-Si). The poly-Si channel is composed of a number of grain boundaries (GB)s, and generated defects due to incomplete atomic bonding or disorder in the GBs create trapping sites, which create space-charge regions between two crystallites that impede the flow of charge carriers from one grain to another. It has been reported that electrical characteristics such as subthreshold swing (S), threshold voltage (Vth), and OFF-state leakage current are degraded by GBs. Moreover, devices having a poly-Si channel show very different I–V characteristics compared with c-Si devices due to the grain boundary and internal grain defects; different temperature dependence characteristics change the Vth distribution of 3D NAND flash memory by affecting the Vth and on-current. On the other hand, 3D NAND flash memory has to stack the channel in a vertical direction to secure bit density and control cost. It is not known whether the channel length is several micrometers or several dozen micrometers for 3D NAND flash memory over 100 layers; as a result, there are many grain boundaries and internal grain defects. Industries assume that SiGe, which contains germanium to improve electron mobility, and GaAs, which is a group III–V compound, are good replacement materials for the poly-Si channel. However, it is not easy to work with a changed channel material and commercialize a product in a short time period. In this thesis, to investigate the cause of changing electrical characteristics in 3D NAND flash memory devices having cylindrical poly-Si channels, I first derived the compact drain-current ID equation of MOSFETs (metal-oxide-semiconductor field-effect transistors) having a cylindrical poly-Si channel, combining the conventional poly-Si-channel 2D TFT (thin film transistor) equation with that of the cylindrical MOSFET. After fitting the simulation data with measured data using the derived drain-current equation, I identified the parameters that could affect the electrical characteristics and temperature. Second, based on the mathematical modeling results, I realized the 3D NAND flash memory using the TCAD simulation tool, verified the fundamental characteristics of poly-Si, and analyzed the influence on Vth, operation current, and S variation by changing the grain boundary potential barrier and adding other parameters under the read cell