Vertical constrained coding for phase-change memory with thermal crosstalk

Phase-change memory (PCM) has been widely considered as one of the most promising non-volatile memory (NVM) technologies due to its high scalability and cost effectiveness. A critical reliability issue caused by PCM cell down scaling is the thermal crosstalk, which leads to the change of a cell´s state while its adjacent cells are being programmed by a high-current reset pulse. In this paper, we propose novel vertical constrained coding scheme in conjunction with parallel programming to mitigate the thermal crosstalk for PCM in both the bit-line (BL) and word-line (WL) directions. We further propose efficient finite-state encoding methods to design capacity-approaching constrained codes which serve as base codes of the vertical constrained coding scheme. Simulation results show that the proposed schemes can effectively reduce the probability of disturbed bits due to thermal crosstalk, and hence help to push the scaling limitation of PCM. It has also been found that the proposed constrained coding scheme can reduce the probability of the reset state, and thereby improve the endurance of PCM.