Electrode roughness and interfacial mixing effects on the tunnel junction thermal stability

The thermal stability of magnetic tunnel junctions with ultrathin (<8 Å) Al2O3 barriers was studied and compared with 15 Å barriers. The tunnel magnetoresistance (TMR) decay cannot be explained only by Mn diffusion into the pinned CoFe layer since this diffusion starts above 300 °C independently of the barrier thickness, while the TMR degradation already occurs at 250–270 °C for the thinner barriers. The thermal stability is probably controlled by changes at the CoFe/Al2O3 interfaces and/or barrier structure. Structural analysis of 15 Å barriers after annealing at 435 °C, shows the existence of an interface region (8–12 Å thick) where CoFe and Al2O3 are found. This interfacial region can be explained by the increased roughness in the bottom electrode after annealing, as measured by atomic-force microscopy (from 1.5 to 4 Å). Ultrathin barriers show a similar trend. The use of low-resistance junctions using thin barriers requires good control of the roughness of the low-resistance bottom electrodes. This is done by preannealing and low-angle ion-beam smoothing 500-Å-thick Cu or Al films, which will then keep a roughness <2 Å during processing temperatures up to 400 °C. Low-resistance junctions (R×A∼40–60 Ω μm2) with 7 Å barriers grown on 600 Å Al buffers after the surface treatment show 25% TMR after annealing at 270 °C.