Thermal Stability Enhancement for Perpendicular-Magnetic Tunnel Junctions Grown on Bcc Crystalline Seed-Layer

Recently, perpendicular-magnetic tunnel junctions (p-MTJs) based on crystalline MgO tunnel barrier has been intensively studied to realize perpendicular-spin-torque-magnetic-random-access-memory (p-STT MRAM). In particular, a TMR of 150 %, thermal stability (Δ) of > 74, and low switching current of for p-MTJs are critical material parameter to achieve a p-STT MRAM. In addition, the integration of p-MTJs with a selective transistor requires a back-of-line of higher than 400 °C. The amorphous CoFeB based p-MTJs fabricated an amorphous Ta seed have met a serious thermal stability degradation when they are ex-situannealed above 325 °C. Up to now, main root-causes of thermal stability degradation have not been clarified. Also, to practicalize p-STT MRAM, the TiN substrate is required as bottom electrode of p-MTJ. The TiN substrate is believed to be a promising bottom electrode for more complicated storage nodes due to reduce a leakage current.

 Thus, in our study, we proposed a novel crystalline heavy metal bcc crystalline seed layer which shows a good thermal stability of amorphous CoFeB based pseudo-spin valve (P-SV) at the ex-situ annealing temperature of above 400 °C. In addition, we investigated the mechanism by which a novel crystalline heavy metal bcc seed layer performs a good thermal stability. Note that in our experiments, we used 12-inch multi-chamber sputter without vacuum breaking during sputtering of p-MTJs. PMA structures grown on Ta or novel bcc crystalline seed layer were shown in Fig. 1(a), where i-PMA was established at the interface between MgO tunneling barrier and amorphous Co₂Fe₆B₂ layer. The saturation magnetization (M_s = 702 emu/cm³) of the Co₂Fe₆B₂ layer grown on amorphous Ta seed layer sustained at ex-situ annealing up to 300 °C and then rapidly degraded when the ex-situ annealing temperature of above 325 °C, as shown in Fig. 1(b). However, the M_s (767 emu/cm³) of the Co₂Fe₆B₂ layer grown on bcc crystalline seed layer abruptly increased from as-sputtered to the ex-situ annealing temperature of 325 °C and then sustained up to the ex-situ annealing temperature of 400 °C. The squareness of the PMA grown on amorphous Ta seed layer degraded at ex-situ annealing temperature of above 350°C, as shown in Fig. 1(c). However, the squareness of the PMA grown on bcc crystalline seed layer show a good squareness, as shown in Fig. 1(d). This result indicates that the Co₂Fe₆B₂ layer grown on bcc crystalline seed layer shows a better PMA characteristic than the Co₂Fe₆B₂ layer grown on amorphous Ta seed layer. For the Co₂Fe₆B₂ layer grown on bcc crystalline seed layer, the dependency of the electrode material on PMA characteristics was shown in Fig. 2. After ex-situ annealing at 400 °C, the Co₂Fe₆B₂layer on bcc crystalline seed layer and TiN/W electrode lost the PMA characteristic, as shown in Fig. 2(a). However, the PMA grown on bcc crystalline seed layer and Ta/TiN/W electrode showed a good PMA characteristic for the bcc crystalline seed layer thickness between 1 and 3 nm, as shown in Fig. 2(b).

The PMA characteristic for P-SV structure grown on bcc crystalline seed layer and Ta/TiN/W electrode demonstrated a typical good PMA characteristic although it was annealed at the ex-situ annealing temperature of 400 °C, as shown in Fig. 3(a). It was confirmed that both amorphous Co₂Fe₆B₂ free and pinned layers in P-SV structure was transferred into well crystalline L₁₀ structure after the ex-situ annealing temperature at 400 °C, as shown in Fig. 3(b). In our presentation, we will review in detail the mechanism by which a novel crystalline heavy metal bcc seed layer performs a good thermal stability. In addition, we will review the TMR ratio with novel crystalline heavy metal bcc seed layer at 400 °C ex-situannealing temperature by using current in plane tunneling (CIPT) method.

* This work was supported by the IT R&D program of MOTIE/KEIT [No.10043398 and 10041608] and the Brain Korea 21 plus Project in 2014, Korea.

Fig. 1. Dependency of Ms and squareness on the seed layer material for PMA structure as a function of ex-situannealing temperature.

Fig. 2. Dependency of squareness on the seed layer material grown on TiN/W electrode for PMA structure, ex-situannealed at 400 °C.

Fig. 3. Squareness and x-TEM image of pseudo-spin valve grown on bcc crystalline seed layer and TiN/W electrode.