A thermally robust perpendicular magnetic tunnel junction with the FeTa decoupling layer

Spin-Transfer-Torque Magnetoresistive Random Access Memory (STT-MRAM) has attracted great attentions as an emerging non-volatile memory. For embedded application, its memory cell i.e., perpendicular magnetic tunnel junction (p-MTJ) needs to withstand the $400 ^{circ}\mathrm {C}$ thermal process which is involved in the semiconductor back-end-of-line (BEOL) process. Although several groups have reported $400 ^{circ}\mathrm {C}$ thermal robustness [1], [2], the key required atomic-level structure for p-MTJs has not yet been clarified. In this work, we have achieved $400 ^{circ}mathrm{Cp} -$MTJ thermal robustness with an untrathin FeTa decoupling layer in the pinned layer, and analysed the atomic-level structure of the film. By comparing the crystal structures of thermally robust and thermally degraded p-MTJs, we have identified that decoupling the body-centered cubic (bcc) CoFeB reference layer and the face-centered cubic (fcc) Co/Pt superlattices is essential in achieving good thermal stability. Firstly, we have confirmed the thermal robustness of a thick p-MTJ against $400 ^{circ}\mathrm {C}$ as reported in Ref. [3]. The p-MTJ structure is Sub/Ta (1.2)/Pt (2.0)/[Co (0.3)/Pt $( 0.5 )] _{6} /$Co (0.5)/ Ru (0.85)/Co (0.5)/Pt (0.5)/[Co (0.3)/Pt $( 0.5 ) ] _{4} /$Ta (0.3)/Co
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Fe $_{60} \mathrm {B}_{20}(1.2) /$MgO(1.0)/Co
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Fe $_{60} \mathrm {B}_{20}(1.2) /$Mo(2.0)/Cap [thickness in nm]. However, for a thinner pinned layer p-MTJ by reducing the Co/Pt repetitions from 4 to 2 labelled as the conventional p-MTJ in Fig. 1(a), thermal stability degradation was observed from the major $M-H$ loops [Fig. 1(b)]. Since the free layer exhibited sufficient thermal stability as shown in the inset of the figure, we speculated that thermal degradation could be initiated from the crystal interference in a thin region of Ta insertion layer where the bcc and fcc crystals meet each other. In order to solve this issue, we proposed a FeTa insertion layer to replace the Ta layer. The improved p-MTJ structure is shown in Fig. 1(a). FeTa was selected due to its amorphous structure as deposited. Under $400 ^{circ}\mathrm {C}$ thermal annealing, it could nucleate and grow in an epitaxial manner along both bcc and fcc crystal boundaries of the neighbouring layers. This unique crystallization behaviour enables FeTa to act as a buffer to effectively decouple the bcc and fcc crystal structures. Regarding the magnetic function, the required ferromagnetic exchange coupling between the CoFeB reference layer and the Co/Pt superlattices was obtained thanks to the intrinsic magnetic property of FeTa alloy. As a result, the thermal robustness against $400 ^{circ}\mathrm {C}$ has been successfully achieved for the improved p-MTJ. Fig. 1(c) shows the decent major and minor $M-H$ loops with little thermal degradation for the improved p-MTJ. To clarify the mechanism for achieving thermal robustness of the improved p-MTJ, we adopted the spherical aberration corrected Scanning Transmission Electron Microscope (Cs-corrected STEM) to visualize the atomic-scale structural difference at the Ta and FeTa interfaces (Fig. 2). Images show that a flat interface is present with the improved FeTa sample, and its CoFeB reference layer is more crystalline structured. In contrast, the conventional Ta sample shows a rougher crystal interface and the CoFeB reference layer tends to be amorphous. The structural difference suggests that the FeTa layer is more effective in decoupling the bcc and fcc crystal structures. This result could also account for the observation by another group that a thicker CoFeB reference layer resulted in thermal stability degradation [2]. Theoretically, a thicker film layer leads to a better thermal tolerance owing to its more robust crystal structure. However, when comes to a complex p-MTJ system consisting of both bcc and fcc lattices in an ultrathin region, a thicker CoFeB layer enhances the structural integrity of bcc lattice against the fcc Co/Pt superlattices. This subtle thickness change may break the thermodynamic equilibrium status between these two lattices during $400 ^{circ}\mathrm {C}$ thermal process, thus causing the degradation of perpendicular magnetic anisotropy originated from the Co/Pt superlattices. This also explained the slightly thermal degradation of the improved p-MTJ when thickening the CoFeB reference layer from 0.8 nm to 1.2 nm even though the FeTa insertion layer can realize more effective crystal decoupling between the bcc CoFeB and fcc Co/Pt superlattices. In summary, maintaining the crystal integrity of both the CoFeB bcc and Co/Pt fcc crystal structures and well control of their interface are essential in achieving good p-MTJ thermal tolerance for embedded STT-MRAM application.