Amorphized Length and Variability in Phase Change Memory Line Cells

The dimensions of amorphized regions in phase-change memory cells are critical parameters to design devices for different applications but are difficult to determine by direct imaging. In this work, the length of amorphized regions in multiple, identical Ge 2 Sb 2 Te 5  (GST) line cells was extracted from electrical measurements. After each cell was programmed to an amorphous state, a sequence of increasing amplitude post-reset voltage pulses separated by low-amplitude read DC-sweeps was applied. When a sufficient amplitude post-reset voltage pulse was applied to a given cell, the measured current and the post-pulse resistance increased drastically, indicating the cell re-amorphized after threshold switching, melting, and quenching. The amorphized length was calculated using the measured voltage at which threshold switching took place and the expected drifted threshold field at that time. The measured threshold voltages, hence, the extracted amorphized lengths, generally increase linearly with the programmed resistance levels, but significant variability arises from the intrinsic uniqueness in the crystallization and amorphization processes in these devices. For example, cells programmed to ~50 MΩ amorphous resistance show threshold voltages of ~5-7 V, corresponding to amorphized lengths of ~240-360 nm. This unpredictable programming feature in phase-change memory devices can be utilized in hardware security applications.