Li, Z., Jeyasingh, R.G.D., Lee, J., Asheghi, M., Wong, H.S.P., and Goodson, K.E., 2012, “Electrothermal Modeling and Design Strategies for Multibit Phase Change Memory,” IEEE Transactions on Electron Devices, Vol. 59, pp. 3561-3567.
Electrothermal transport and crystallization dynamics govern the speed and bit stability of multibit phase change memory (PCM). This work develops a transient simulation methodology incorporating electrical, thermal, and phase transition models to investigate multibit PCM cell structures and programming strategies. The simulations evaluate two standard PCM structures, the mushroom cell and the confined pillar cell, with feature sizes smaller than 40 nm. The transient simulation captures the phase distribution and cell resistance profile, which are corroborated by transmission electron microscope (TEM) imaging and the corresponding measured resistance values. This work also explores a more compact architecture, the stacked vertical cell, with precise control of the Joule heating and potentially more stable intermediate resistance levels. For an electrode area of 10 nm × 20 nm, a low programming current of 60 μA – 90 μA generates sufficient heating power to amorphize the phase change elements sequentially, resulting 4 distinct resistance levels distributed over a 2-order-of-magnitude resistance range with promise for multibit storage.