Effect of coupling ability between a synthetic antiferromagnetic layer and pinned layer on a bridging layer of Ta, Ti, and Pt in perpendicular-magnetic tunnel junctions

Abstract

By fabricating CoFeB/MgO/CoFeB-based perpendicular-magnetic tunnel junction (p-MTJ) spin-valves stacked with a [Co/Pd](n)-SyAF layer based on a TiN bottom electrode on a 12 inch Si wafer (001) substrate, we investigated how the bridging layers of Ta, Ti, and Pt and their thickness variation affected the tunneling magneto-resistance (TMR) ratio of Co2Fe6B2 pinned-layer behavior in magnetic-tunnel-junctions. TMR ratios for Ta, Ti, and Pt bridging layers were observed to be 64.1, 70.2, and 29.5%, respectively. It was confirmed by high resolution transmission electron microscopy (HR-TEM) that this difference resulted from CoFeB/MgO/CoFeB MTJ layers with Ta and Ti bridging layers being textured well with a bcc (100) structure, indicating that Ta and Ti bridging layers bridged SyAF fcc (111) and MTJ bcc (100). On the other hand, the MTJ layer with Pt bridging layer was incorrectly textured, indicating that a Pt bridging layer is unsuitable to bridge SyAF fcc (111) and MTJ bcc (100) due to Pt being diffused into the CoFeB pinned-layer. In addition, perpendicular magnetic anisotropy (PMA) behavior of the CoFeB pinned-layer was found to depend strongly on a bridging layer thickness; higher TMRs of Ta and Ti were observed at the optimal bridging layers' thickness, which enable the realization of PMAs of the pinned-layer and ferro-coupling of the pinned-layer with the lower-SyAF layer. Among the three bridging materials (Ta, Ti, and Pt), we observed that Ti showed the highest TMR ratio and widest thickness range for a high TMR ratio, indicating that a higher TMR ratio is needed to obtain the best deposition process margin.