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

The thermal stability of magnetic tunnel junctions with ultrathin (<8 Angstrom) Al2O3 barriers was studied and compared with 15 Angstrom barriers. The tunnel magnetoresistance (TMR) decay cannot be explained only by Mn diffusion into the pinned CoFe layer since this diffusion starts above 300 degreesC independently of the barrier thickness, while the TMR degradation already occurs at 250-270 degreesC 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 Angstrom barriers after annealing at 435 degreesC, shows the existence of an interface region (8-12 Angstrom 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 Angstrom). 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-Angstrom -thick Cu or Al films, which will then keep a roughness <2 Angstrom during processing temperatures up to 400 degreesC. Low-resistance junctions (R x A similar to 40-60 Ohm mum(2)) with 7 Angstrom barriers grown on 600 Angstrom Al buffers after the surface treatment show 25% TMR after annealing at 270 degreesC. (C) 2001 American Institute of Physics.