High resolution electron microscopy observation of different Al-oxide layers in magnetic tunnel junctions

Microstructure and morphology of Al-oxide layers in magnetic tunnel junctions (MTJs) were characterized using high resolution electron microscopy (HREM). The MTJs were fabricated using magnetron sputtering. The Al-oxide layers were prepared using four different methods; natural, thermal, and plasma oxidation techniques as well as sputtering from pure alumina. HREM results showed that sputtering conditions, surface roughness of the bottom electrode in the MTJs, and oxidation conditions for the Al-oxide are essential factors affecting the quality of the insulating layer. High sputtering power was required to deposit a relatively smooth layer of pure alumina. At matrices were observed in Al-oxide with only 24 h natural oxidation time. It was found optimum to thermally oxidize an Al-oxide layer between 333 and 473 K for 48 h. It is more effective and useful to oxidize an Al layer at a slightly higher plasma power rather than using a longer oxidation time in plasma oxidation method.