Development of spin funneling magnetic field sensor devices for high sensitive and high space-resolution magnetic imaging plates

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In order to realize a high sensitive and high space-resolution magnetic imaging plate, we investigated fabrication processes of spin tunneling magnetic field sensor devices. With the microwave excited plasma oxidizatioin or nitridation method using radial line slot antenna (RLSA) irradiator, ferromagnetic tunnel junctions (MTJs) in stack of bottom-electrode/Mn-Ir/Co-Fe/barrier/Co-Fe/Ni-Fe/upper-electrode were fabricated on a thermally oxidized Si wafer. The barrier was formed by the plasma oxidization or nitridation process with X+O_2 or X+N_2 (X=He,Ar,Kr) mixed gases, immediately after the deposition of 0.8-1.5 nm-thick metal Al films. In the case of MTJs with Al-O barrier oxidized from 1.5-nm-thick Al film, the maximum TMR ratio was achieved to 58.8%, which was the world record at the time. In the case of MTJs with Al-N barrier nitrided from 1.0-nm-thick Al film, the maximum TMR ratio overcame the conventional record of 33% and reached 49%. It was also found that plasma nitridation of metal Al films progresses more mildly than the plasma oxidization of them. This means that the plasma nitridation is suitable for the formation process of ultra-thin barrier layers. High concentration ozone oxidization process of ultra-thin metal films and magnetotransport properties of MTJs fabricated with this process were also investigated. As results followings were found. The oxidization process of metal films are different between the ozone exposure method and the plasma exposure method. The oxidizing rate (parabolic rate constant, k_p), dominated by the diffusion velocity of oxidizing species in the oxide layer, is smaller by two-orders in magnitude for the ozone process than that for the plasma process, while the thickness of oxide layer, immediately formed on the surface at the very early stage of oxidization, is almost same. This means that the high concentration ozone oxidization method is suitable for the precise control of oxidization of ultra-thin metal films.