Effect of Hydrogen Profile in Flash Memory SiNx Charge Trap Layer with Different Silicon to Nitrogen Ratios

Abstract

As the number of word-line layers of vertical flash memory increases, it is difficult to develop high aspect ratio contact further. NAND cell scaling can consistently reduce with advanced fabrication development, but the reliability deterioration becomes challenge as the cell-to-cell distance decreases. In this study, the hydrogen profile in the SiO2/SiNx/SiO2 (ONO) stack is controlled through post annealing treatment and forming accessible deep level traps. When ONO stack employing with SiNx(x:1.02) is N2-annealed, Si─Si and Si-dangling bonds are observed. The polaron effect stemming from the Si─Si bonds led to a reduction in charge loss, thereby maintaining 84% of the memory window (MW). Conversely, when ONO stack employing SiNx(x:1.24) is annealed under forming gas ambient, the MW is increased from 4.68 to 6.57 V. This is attributed to the passivation of interface trap by dissociating N─H bonds and alleviating charge retention by reduction in the density of Si-dangling bond, leading to maintaining 89.7% of MW. These results address the reliability issue caused by trapped-charge instability and successfully mitigate the trade-off relation between MW and retention characteristics.