The impact of heater-recess and load matching in phase change memory mushroom cells

Two-dimensional finite element simulations with rotational symmetry are used to analyze the impact of the bottom electrode recess on the reset operation of phase change memory elements with mushroom cell geometry (Ge2Sb2Te5 (GST) film over a patterned TiN pillar). Temperature dependent materials parameters are used for GST and TiN, and the latent heat of fusion in melting of GST is included to model melting. The results of this study indicate that a lower reset current and a more favorable thermal profile may be achieved by extending the active region of GST down into the pillar, due to the heat confinement. It is shown that the current through cells with an insufficient load condition for maximum power transfer can be maintained at a level lower than that which is sufficient for reset operation for extended periods of time due to the non-linear nature of temperature dependent electrical conductivity of GST. These results suggest that if the load condition is not matched, excessive voltage levels or pulse durations would be necessary to achieve successful reset operation across cell arrays.