A quasi-analytical model for energy-delay-reliability tradeoff studies during write operations in perpendicular STT-RAM cell

Abstract

One of the biggest challenges the current STT-RAM industry faces is maintaining a high thermal stability while trying to switch within a given voltage pulse and energy cost. In this paper, we present a physics based analytical model that uses a modified Simmons' tunneling expression to capture the spin dependent tunneling in a magnetic tunnel junction(MTJ). Coupled with an analytical derivation of the critical switching current based on the Landau-Lifshitz-Gilbert equation, and the write error rate derived from a solution to the Fokker-Planck equation, this model provides us a quick estimate of the energydelay- reliability tradeoffs in perpendicular STTRAMs due to thermal fluctuations. In other words, the model provides a simple way to calculate the energy consumed during a write operation that ensures a certain error rate and delay time, while being numerically far less intensive than a full-fledged stochastic calculation. We calculate the worst case energy consumption during anti-parallel (AP) to parallel (P) and P to AP switchings and quantify how increasing the anisotropy field HK and lowering the saturation magnetization MS, can significantly reduce the energy consumption. A case study on how manufacturing variations of the MTJ cell can affect the energy consumption and delay is also reported.