A comprehensive study on interface perpendicular MTJ variability

Spin transfer torque MRAM (STT-MRAM) is one of the promising candidates as a scalable nonvolatile memory with high density, and CMOS compatibility [1], [2]. Interface perpendicular magnetic tunnel junction (PMTJ) shown in Fig. 1 has been demonstrated with the goal of reducing the switching current while maintaining sufficient nonvolatility [3]. However, previous studies report that PMTJ suffers from process-dependent dimensional variations, thus it remains one of the major constrains in achieving high performance STT-MRAM [4, 5]. As shown in the equations of Fig. 1, the anisotropy field (H_K) and free layer volume (V) are functions of PMTJ dimensions, hence their variations result in variation of STT switching characteristics such as thermal stability factor (Δ) and switching current (I_C). The H_K of PMTJ has a strong dependency on relative ratio between the free layer thickness (t_F) and the critical thickness (t_C) [3]. The equations of Fig. 1 suggest that the t_F variation differently affects the PMTJ dimension-dependent parameters (gray circles), resulting in either increasing or decreasing Δ and/or I_C. This paper presents a comprehensive study on process-dependent dimensional variability of PMTJ, especially focusing on estimating the impact of t_F variation on Δ and I_C variability. For a practical analysis, our physics-based macrospin SPICE model [7] captures the key physics of STT switching in PMTJ by incorporating all of the above mentioned PMTJ dimension-dependent parameters into the Landau-Lifshitz-Gilbert (LLG) equation.