Investigation of Abnormal $V_{rm TH}/V_{rm FB}$ Shifts Under Operating Conditions in Flash Memory Cells With $ hbox{Al}_{2}hbox{O}_{3}$ High-$kappa$ Gate Stacks

摘要

$hbox{Al}_{2}hbox{O}_{3}$ high- $kappa$ stack is a strong candidate as the dielectric layer in Flash memory cells for technology generations beyond sub-20 nm. In this paper, the cause of abnormal $V_{rm TH}/V_{rm FB}$ shift at low operating electric fields is investigated, i.e., $V_{rm TH}/V_{rm FB}$ reduces at low positive gate biases and increases at low negative gate biases. It is found that this instability does not originate from the electrons trapping/detrapping from the gate nor from the dielectric relaxation. For the first time, extensive experimental evidences show that this shift is caused by as-grown mobile charges in $hbox{Al}_{2}hbox{O}_{3}$ layers generated by postdeposition annealing at 1000 $^{circ}hbox{C}$ or above. Its impacts on program/erase windows and read/pass disturbance in Flash memory cells are also evaluated.