Challenging Issues for Terabit-Level Perpendicular STT-MRAM

Recently, many researchers have intensively studied p-STT MRAM as an alternative terra-bit-integration nonvolatile memory since current Si-based dynamic-random-access-memory has a physical scaling limit of less than 20 nm. p-STT MRAM  has demonstrated a fast write time of ~10 ns, non-volatile memory operation, and extremely low power consumption. However, for satisfying a commercial specification as a terra-bit-integration nonvolatile memory, perpendicular magnetic tunneling junction (p-MTJ) of p-STT MRAM essentially needs to satisfy critical device performances for p-MTJ spin-valves such as a high TMR ratio of 150 %, a high J_ex of > 0.7 erg/cm², a good K_u of 1x10⁷ erg/cm³, a good �� of ~ 74, and a low critical writing current (J_C0) of 13.4 MA/cm², which should satisfy the thermal stability such as an ex-situ annealing above 400 °C. In general, a CoFeB based p-MTJ spin-valve in p-STT-MRAM has been fabricated with the stacked structure of bottom electrode/ amorphous Ta seed layer/amorphous CoFeB free layer/MgO tunneling barrier/amorphous CoFeB pinned layer/capping layer/buffer layer/ synthetic anti-ferromagnetic layer (SyAF) /top electrode. In particular, nano-scale thick amorphous CoFeB layer has been applied for free and pinned layer. However, the mechanism by which the perpendicular magnetic anisotropy (PMA) characteristic for amorphous CoFeB based p-MTJ using amorphous Ta seed layer could be achieved has not been clarified. In our study, we will review its mechanism and the thermal stability limit for amorphous CoFeB based p-MTJ using amorphous Ta seed layer. In addition, we will present the thermal stability enhancement for amorphous CoFeB based p-MTJ using bcc crystalline seed layer and its mechanism. For amorphous CoFeB based p-MTJ using amorphous Ta seed layer, the B atoms were segregated at the interface between the MgO tunneling barrier and the CoFeB free and pined layer after the ex-situ annealing of 275 °C, producing interface-PMA (i-PMA), as shown in Fig. 1(a). However, the PMA characteristics for amorphous CoFeB based p-MTJ using amorphous Ta seed layer was rapidly degraded as the ex-situ annealing temperature increased above 400 °C, which the mechanism was not proved yet. Surprisingly, the crystalline structure of both CoFeB free layer and pinned layer sustained at the amorphous structure although the p-MTJ was ex-tu annealed at 275 °C, as shown in Fig. 2.

 In order to enhance the thermal stability of amorphous CoFeB based p-MTJ, a bcc crystalline seed layer is essentially necessary rather than amorphous Ta seed layer. During ex-situ annealing at 400 °C, the bcc crystalline seed layer could texture the amorphous CoFeB free and pinned layer into the bcc crystalline CoFe free layer, MgO tunneling, and CoFeB pinned layer, as shown in Fig. 3. As a result, amorphous CoFeB based p-MTJ could achieve the TMR of ~135 %, as shown in Fig. 4. In this presentation, I will challenging material issues for p-STT MRAM in the view of the perfect crystalline linearity for free layer, MgO tunneling barrier, and pinned layer, and SyAF layer.