Thermal Stability Enhancement for Perpendicular-Magnetic Tunnel Junctions Grown on Bcc Crystalline Seed-Layer
Authors: Lee Du-Yeong , Jeon Min-Su , Hong Jin-Pyo , Park Jea-Gun 
Published: February 2020

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 (Ms = 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 Ms (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.