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Al2O3 high-κ 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 VTH/VFB shift at low operating electric fields is investigated, i.e., VTH/VFB 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 Al2O3 layers generated by postdeposition annealing at 1000 °C or above. Its impacts on program/erase windows and read/pass disturbance in Flash memory cells are also evaluated.
Date of Publication: July 2012