Improvement in performance and Reliability of a-InGaZnO thin-film transistor with iCVD polymer fluorine doping

Abstract

Due to the recent development of the 4th industry, the demand for memory semiconductors is increasing rapidly. Accordingly, in the case of flash memory, a cell-on-peri (COP) structure for stacking peripheral circuits at the bottom of a three-dimensional memory cell is being newly developed, and in the case of DRAM, a monolithic 3D integrated structure for stacking upper transistors on top of lower transistors is drawing attention as a new technology. However, in order to form a transistor at the top, low heat treatment constraints are inevitable to minimize damage to the lower device. Existing polysilicon channels require a high deposition temperature of $630^\circ C$ and subsequent heat treatment of $900^\circ C$ or more for doping, and there is a problem that mobility is reduced due to deterioration in the grain boundary. On the other hand, oxide semiconductors are expected to be the most suitable material for channels in 3D stacked structures because they are amorphous but have the advantages of high mobility, low leakage current, and deposition at room temperature. However, oxide semiconductors, as diversified ion- bonding materials, primarily face challenges of performance degradation and electrical instability issues caused by oxygen defects. Previously, oxygen atmosphere heat treatment or oxygen-based plasma surface treatment doping has been improved, but doping of other elements has emerged as oxygen rapidly reduces the number of electrons in the channel and reduces electrical performance. Therefore, in this paper, fluorine in an oxide semiconductor (a-InGaZnO) was successfully doped using an integrated-Chemical-Vapor-Deposition (iCVD)-based fluorine polymer process. Fluorine has more electrons than oxygen, which is group 17, and has improved performance and stability because it forms strong ionic bonds based on high electronegativity. In addition, unlike conventional doping methods (plasma, ion-implant), the iCVD polymer doping method can minimize surface damage, thereby securing process stability. Finally, through double layer passivation, deterioration caused by the external environment could be suppressed, thereby maximizing electrical stability.