InGaZnO based charge trap device for NAND flash memory application

Abstract

Current information technology industry requires faster, smaller, less power consuming and cost effective memory devices, which is the fundament of the computer system. Out of many memory devices, NAND flash memory technology one of the promising candidate for next generation storage system. Planar technology is at its roadblocks due to the fundamental limitation of the density per available wafer area. Hence, vertical NAND flash (VNAND) is the solution, which can overcome the density limitation. Current industry use three major types of V-NAND (i.e. BiCs, TCAT and Pipe-BiCs). These major architectures of V-NAND all require Etch hole, where ONO, channel and oxide is deposited. Due to limitation on wafer area, etch hole and spacer between etch holes have limitations, often called as the critical diameter or dimension. In case of etch hole itself, each packing density has different critical diameter of hole (CD). Currently, a-Si or poly-Si channels are used as the channel material. However, as etch hole depth elongates, to increase cell density, Si based channel suffer from low on current, which can be very problematic in achieving constant device uniformity. To increase on current, Si based channel requires thickening, but again, limited by the CD. Hence, new channel material that can guarantee tunable electrical characteristic is ideal. Out of many candidates, amorphous oxide is one of the promising candidate. InGaZnO (α-IGZO or IGZO) has been well studied to be applied on V-NAND technology. Professor Hosono first proposed IGZO in the year 2004. Since then, IGZO has been intensely studied for its application on display devices. With most common deposition technique, sputtering. Large area, fast deposition rate, and relatively uniform film quality had lead such popularity. In primitive stage of the IGZO research, most problematic characteristic was materials instability against light illumination under negative bias (NBIS). When light wavelength of 550nm (or shorter) is induced on IGZO (held at negative gate bias), negative shift of the transfer curve was observed. Numerous arguments have been proposed as a possible cause, but most agreed that it was the ionization of the oxygen vacancy. When light is induced, the oxygen vacancy (naturally occurring from amorphous large band gap nature of the material) begins to ionize, changing its valance state from neutron to two (creating two extra electron). Previous research from our group has shown that NBIS is geometry dependent (NBIS worsen as channel length increase), hence, smaller channel dimension may solve the issue. Other possible solutions were to doping and use passivation layer. Despite its intensive study for display application, IGZOs application on V-NAND still requires major challenge, to change deposition technique. Despite fast and low temperature merits of the sputtering process, its low step coverage hinders IGZO for the memory applications. In this sense, this research proposes relatively new deposition technique to deposit IGZO: metal-organic chemical vapor deposition (MOCVD). First part of this research compares program and erase characteristics of MOCVD and sputter deposited a-IGZO. Both deposition methods have shown to have similar atomic percent (stoichiometry), density and thickness. Despite, slightly higher saturation mobility from sputter IGZO, the program and erase characteristics showed much faster operation in case for the MOCVD IGZO (PGM 3.12V @ VPGM=20V,tPGM=100ms for MOCVD and PGM 1.63V @ VPGM=20V, tPGM=100ms). In depth analysis of such behavior were studied. Second part concerns the composition variation of the IGZO. MOCVD processed IGZO were studied with different Ga/Zn ratio, keeping the In content at constant (40 atomic percent). Previous research has indicated Ga as the carrier sink, allowing the stable off current of the TFT, due to strong bonding between Ga and O. However, study on different Ga/Zn ratio has indicated while highest field effective channel mobility was observed at max Zn content (of the study), highest saturation mobility was observed when Ga/Zn ratio was at maximum (of the study). Analysis to understand the role of Ga/Zn within the IGZO thin film is further investigated