Highly scalable vertical bandgap-engineered NAND flash memory

Abstract

In this work, highly scalable charge trap flash (CTF) memory with bandgap-engineered storage node and vertical channel is proposed. Due to the compact cell layout without individual junction contacts, NAND flash memory has the most suitable architecture for mobile data storage media. In other to achieve even higher integration density, two NAND flash memory cells in the conventional sting are put together to have a common vertical channel as shown in Fig. 1(a). The biggest merit in this array feature is that half-level reduction in footprint can be achieved by using the sidewall control gates as shown in Fig. 1(b). Due to the vertical channel, the channel length can be simply controlled by anisotropic dry etch, which suppresses short channel effects (SCEs) effectively and enlarges the sensing margin. Fig. 1(c) shows the schematic view of the array with circuit symbols. Also, in this novel flash memory, the memory storage node uses a bandgap-engineered (BE) multi-layer, where the oxide-nitride-oxide triple-layer replaces the tunnel oxide to improve the performances in operations [1–3]. The process architecture for fabricating the vertical BE-NAND flash memory is summarized in Fig. 2. The fin structures in both bitline (BL) and wordline (WL) directions have been formed by sidewall spacer patterning [4]. Among the several possible combinations of materials [5–6], nitride (Si3N4) and TEOS are used for the sidewall spacer and the supporting dummy pattern, respectively. Fig. 3 and 4 shows the process flow for Si fin constructions in both directions. After the fin consisted of Si-STI alternating pillars is formed in the WL direction, ONONO (20/20/20/60/60 Å) multi-layer is deposited. All the oxide layers were deposited by medium temperature oxidation (MTO) in an ambient of N2 160 sccm and DCS 40 sccm. The tunneling and storage nitride layers were deposited by LPCVD in the ambient of NH3 30 sccm/DCS 10 sccm/750 °C and NH3 100 sccm/DCS 30 sccm/785 °C, respectively. Subsequently, physically separated sidewall control gates used as WLs are formed by etch-back process. Fig. 5(a) through (c) show the images for fin structures and top view of the vertical BE-NAND flash with independent sidewall gates. After ILD/CMP/metallization/alloy, the fabrication is completed.