Investigation of the Connection Schemes between Decks in 3D NAND Flash

Dual-deck stacking technology is an effective solution for solving the contradiction between the demand for increasing storage layers and the challenge of the deep hole etching process in 3D NAND flash. The connection scheme between decks is a key technology for the dual-deck structure. It has become one of the necessary techniques for 3D NAND flash storage density improvement. This article mainly studies the impact of connection schemes between decks on cell reliability. Based on experimental data and simulation analysis, unfavorable effects were found as the gate channeling the breakdown and data retention characteristics of the top cells in the lower deck deteriorated due to the local electric field enhancement in the connection scheme without a poly-plug. This mainly contributed to the structural change of these cells within process impact. They will suffer secondary etching during the upper deck channel etching process due to alignment issues between the upper and lower decks. In another scheme with a poly-plug connection between decks, the saturation current of the channel decreased and the current variation increased. The fundamental cause of the current anomaly is that the Poly-plug has a certain shielding effect on channel inversion and the weak inversion region becomes a bottleneck for the channel current. The increase in variation is due to the shielding effect differences in the different structures of the poly-plug. Therefore, for the connection scheme without a poly-plug, the article proposes to improve device reliability by increasing the oxide thickness between decks and setting the top cells of the lower decks to be virtual cells. For the connection scheme with a poly-plug, the plug's N-type doping scheme is proposed to avoid the current dropping anomaly.