Enhancing Programming Variability in Multi-Bit Phase Change Memory Cells for Security

We performed gradual programming of phase change memory (PCM) line cells with repeated pulses to transition from the crystalline to the amorphous state through various intermediate states and analyzed the variability in programming trends for different cell shapes and sizes. The crystalline PCM cells experienced sequences of repeated applied voltage pulses of either identical amplitudes or gradually increasing amplitudes. With repeated identical amplitude pulses, large number of pulses (~1,000) were required to reach a full reset of the cells through partial reset and partial set steps. With repeated increasing amplitude pulses, ~20-30 pulses were sufficient to fully reset the cells through gradual partial reset steps. Both programming approaches resulted in significant cell-to-cell variability for all cells, with stronger variability for narrower cells. However, the latter pulsing approach was more practical and resulted in more uniform distribution of all states. The stronger cell-to-cell programming variability for the narrower cells is attributed to the smaller number of crystalline grains in each cell and to the larger relative impact of manufacturing process variations. Statistical analysis of two example data sets shows promise of PCM programming variability for hardware security applications.