Mobile-Ion-Induced Charge Loss Failure in Silicon–Oxide–Nitride–Oxide–Silicon Two-Bit Storage Flash Memory

Abstract

In silicon–oxide–nitride–oxide–silicon (SONOS) 2-bit storage flash memory, we discovered deterioration of data retention (DR) in the form of charge loss, which is dependent on the distance between contact windows and word lines (WLs) and also on the thermal treatment performed after the formation of contact windows. We hypothesized that the unique structure of the SONOS flash memory leads to susceptibility to mobile ion contamination. We concluded that Na mobile ion contamination originates in the tungsten chemical–mechanical polishing (W-CMP) process, and that the ions diffuse through the boundaries of the boron phosphosilicate glass (BPSG) and stacked oxide–nitride–oxide (ONO) films into the cell area. We successfully reduced the charge loss by cleaning of the contamination source and by the stable control of phosphorus concentration at the bottom of the BPSG. As a permanent countermeasure, we proposed the complete isolation of contact windows from the adjacent ONO layer, and we were able to demonstrate the effectiveness of this proposal.