Data retention failure in NOR flash memory cells

We present the results of investigations into the causes of threshold voltage instabilities in NOR-type flash memory cells due to charge loss and charge gain. A large threshold voltage shift of several volts has been observed on specific cells, which have a bit line contact that is misaligned and touches the side wall spacer. This data retention failure is characterized by both a temperature dependence of charge loss, showing an activation energy of about 1.12 eV, and a dependence of the magnitude of charge gain on the charged state in previous retention bake stressing. Employing a thin silicon nitride layer between the side wall spacer and the bit line contact results in good data retention. Based on these results, sodium ion contamination is proposed as the origin of charge loss and charge gain in NOR-type flash cells fabricated using a non-optimized process.