Design of projected phase-change memory mushroom cells for low-resistance drift

Projected phase-change memory (PCM) devices have been proposed as a solution to the challenge of resistance drift, an issue where PCM cell resistance increases as a function of time. Here, we theoretically and experimentally study the performance of projected mushroom PCM cells. Using circuit models, we show that the effective drift coefficient of projected PCM cells is proportional to the fraction of the current flowing through the drifting material. To further characterize device operation, we utilize a finite element model and find that tuning the projection liner sheet resistance enables a dynamic range, the ratio of the RESET resistance to the SET resistance, of 14 with a thin 2-nm projection liner. Additionally, we show that increasing the doping level provides a useful tuning knob to increase SET and RESET resistance without sacrificing dynamic range or drift performance. We fabricate projected PCM mushroom cells on 300-mm wafers to calibrate the model parameters and find that experimental trends are consistent with these theoretical predictions.