In this work, the influence of BEOL process on poly-Si grain boundary traps passivation in 3D NAND flash is investigated. Two hydrogenation methods in final passivation process are compared and hydrogen in passivation layer is found to be more effective for poly-Si grain boundary traps (GBT) passivation, according to device characteristics. Interlayer used as copper cap layer can also act as potential hydrogen diffusion source as well as final passivation layer. Besides, different interlayer films in BEOL process are found to be critical to cell device characteristics. It is considered that BEOL film stacks can influence poly-Si GBT density and cell device characteristics during subsequent hydrogen passivation process.