Characterization of CoFeB electrodes for tunnel junctions

This paper compares structural and magnetic properties of CoFeB thin films and tunnel junction electrodes, with a boron content of 10 at % (CFB10) and 20 at % (CFB20). X-ray diffraction of 20–60 Å thick CoFeB films indicates amorphous structure in the as-deposited state, independent of the B content. The CFB10 films develop a strong (111) texture after annealing at 280 °C, while CFB20 films require annealing at 320 °C. However, films with either composition can remain amorphous upon anneal, if thinner than 40 Å. The crystallization temperature was corroborated by analysis of exchange bias and coercive fields. Tunnel junctions based on CFB10 and CFB20 were fabricated by ion beam and magnetron sputtering, respectively, and patterned down to 1×2μm2. From magnetic measurements, bottom-pinned MnIr∕CFB10 or MnIr∕CFB20 junctions have similar exchange fields upon anneal. For top-pinned structures, week exchange is obtained using CFB20∕MnIr. Synthetic antiferromagnets (CFB∕Ru∕CFB) were also studied. Antiferromagnetic coupling (AF) can be produced with 6–8 Å thick Ru spacers in glass∕CFB10∕Ru∕CFB10 multilayers, while CFB20∕Ru∕CFB20 amorphous structures are only weekly AF coupled. CFB10-based junctions can sustain annealing up to 360 °C without degradation of the magnetic properties of the CFB electrodes and ferromagnetic coupling fields (Hf), while maintaining TMR values of 30%.