Thermal analyses of confined cell design for phase change random access memory (PCRAM)

This investigation is an effort to demonstrate the feasibility of confined cell design for a phase change random access memory (Ovonic Unified Memory, OUM) cell which benefits, respectively, from the low current and the simple fabrication process of a line memory cell and T- structure. Similar to OUM devices, the confined cell design is also highly scalable. The 2-D numerical simulation results of the programming process for confined cells show very high (104 - 105) reset/set resistance ratios. However, the proper selection of the reset current is very important for achieving a complete amorphization of the active region, free of crystalline regions. Formation of such crystalline residues can create low resistance paths for the electric current rendering unacceptably low reset/set resistance ratios (of order two) during read. Simple analytical expressions for the reset current and thermal tome constants of a confined cell were obtained using 2-D and 3-D heat conduction heat transfer analyses. The results agree well with those of 2-D thermal and crystallization modeling, which indicate that the analytical models can be used as a powerful preliminary design tool for PCRAM.